--- a/devtools/client/debugger/jest-test.config.js
+++ b/devtools/client/debugger/jest-test.config.js
@@ -35,16 +35,17 @@ module.exports = {
   setupFiles: ["<rootDir>/src/test/shim.js", "jest-localstorage-mock"],
   snapshotSerializers: [
     "jest-serializer-babel-ast",
     "enzyme-to-json/serializer",
   ],
   moduleNameMapper: {
     "\\.css$": "<rootDir>/src/test/__mocks__/styleMock.js",
     "\\.svg$": "<rootDir>/src/test/__mocks__/svgMock.js",
+    "devtools-services": "<rootDir>/src/test/fixtures/Services",
     "^Services": "<rootDir>/src/test/fixtures/Services",
     "^chrome": "<rootDir>/src/test/fixtures/Chrome",
     "^ChromeUtils": "<rootDir>/src/test/fixtures/ChromeUtils",
     "\\/plural-form$": "<rootDir>/src/test/fixtures/plural-form",
     "shared\\/telemetry$": "<rootDir>/src/test/fixtures/telemetry",
     "\\/unicode-url$": "<rootDir>/src/test/fixtures/unicode-url",
     // Map all require("devtools/...") to the real devtools root.
     "^devtools\\/(.*)": "<rootDir>/../../$1",

--- a/devtools/client/debugger/src/test/fixtures/Services.js
+++ b/devtools/client/debugger/src/test/fixtures/Services.js
@@ -1,11 +1,533 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 "use strict";
 
-module.exports = {
+/* globals localStorage, window, document, NodeFilter */
+
+// XXX: This file is a copy of the Services shim from devtools-services.
+// See https://github.com/firefox-devtools/devtools-core/blob/a9263b4c3f88ea42879a36cdc3ca8217b4a528ea/packages/devtools-services/index.js
+// Many Jest tests in the debugger rely on preferences, but can't use Services.
+// This fixture is probably doing too much and should be reduced to the minimum
+// needed to pass the tests.
+
+// Some constants from nsIPrefBranch.idl.
+const PREF_INVALID = 0;
+const PREF_STRING = 32;
+const PREF_INT = 64;
+const PREF_BOOL = 128;
+const NS_PREFBRANCH_PREFCHANGE_TOPIC_ID = "nsPref:changed";
+
+// We prefix all our local storage items with this.
+const PREFIX = "Services.prefs:";
+
+/**
+ * Create a new preference branch.  This object conforms largely to
+ * nsIPrefBranch and nsIPrefService, though it only implements the
+ * subset needed by devtools.  A preference branch can hold child
+ * preferences while also holding a preference value itself.
+ *
+ * @param {PrefBranch} parent the parent branch, or null for the root
+ *        branch.
+ * @param {String} name the base name of this branch
+ * @param {String} fullName the fully-qualified name of this branch
+ */
+function PrefBranch(parent, name, fullName) {
+  this._parent = parent;
+  this._name = name;
+  this._fullName = fullName;
+  this._observers = {};
+  this._children = {};
+
+  // Properties used when this branch has a value as well.
+  this._defaultValue = null;
+  this._hasUserValue = false;
+  this._userValue = null;
+  this._type = PREF_INVALID;
+}
+
+PrefBranch.prototype = {
+  PREF_INVALID: PREF_INVALID,
+  PREF_STRING: PREF_STRING,
+  PREF_INT: PREF_INT,
+  PREF_BOOL: PREF_BOOL,
+
+  /** @see nsIPrefBranch.root.  */
+  get root() {
+    return this._fullName;
+  },
+
+  /** @see nsIPrefBranch.getPrefType.  */
+  getPrefType: function(prefName) {
+    return this._findPref(prefName)._type;
+  },
+
+  /** @see nsIPrefBranch.getBoolPref.  */
+  getBoolPref: function(prefName, defaultValue) {
+    try {
+      let thePref = this._findPref(prefName);
+      if (thePref._type !== PREF_BOOL) {
+        throw new Error(`${prefName} does not have bool type`);
+      }
+      return thePref._get();
+    } catch (e) {
+      if (typeof defaultValue !== "undefined") {
+        return defaultValue;
+      }
+      throw e;
+    }
+  },
+
+  /** @see nsIPrefBranch.setBoolPref.  */
+  setBoolPref: function(prefName, value) {
+    if (typeof value !== "boolean") {
+      throw new Error("non-bool passed to setBoolPref");
+    }
+    let thePref = this._findOrCreatePref(prefName, value, true, value);
+    if (thePref._type !== PREF_BOOL) {
+      throw new Error(`${prefName} does not have bool type`);
+    }
+    thePref._set(value);
+  },
+
+  /** @see nsIPrefBranch.getCharPref.  */
+  getCharPref: function(prefName, defaultValue) {
+    try {
+      let thePref = this._findPref(prefName);
+      if (thePref._type !== PREF_STRING) {
+        throw new Error(`${prefName} does not have string type`);
+      }
+      return thePref._get();
+    } catch (e) {
+      if (typeof defaultValue !== "undefined") {
+        return defaultValue;
+      }
+      throw e;
+    }
+  },
+
+  /** @see nsIPrefBranch.getStringPref.  */
+  getStringPref: function() {
+    return this.getCharPref.apply(this, arguments);
+  },
+
+  /** @see nsIPrefBranch.setCharPref.  */
+  setCharPref: function(prefName, value) {
+    if (typeof value !== "string") {
+      throw new Error("non-string passed to setCharPref");
+    }
+    let thePref = this._findOrCreatePref(prefName, value, true, value);
+    if (thePref._type !== PREF_STRING) {
+      throw new Error(`${prefName} does not have string type`);
+    }
+    thePref._set(value);
+  },
+
+  /** @see nsIPrefBranch.setStringPref.  */
+  setStringPref: function() {
+    return this.setCharPref.apply(this, arguments);
+  },
+
+  /** @see nsIPrefBranch.getIntPref.  */
+  getIntPref: function(prefName, defaultValue) {
+    try {
+      let thePref = this._findPref(prefName);
+      if (thePref._type !== PREF_INT) {
+        throw new Error(`${prefName} does not have int type`);
+      }
+      return thePref._get();
+    } catch (e) {
+      if (typeof defaultValue !== "undefined") {
+        return defaultValue;
+      }
+      throw e;
+    }
+  },
+
+  /** @see nsIPrefBranch.setIntPref.  */
+  setIntPref: function(prefName, value) {
+    if (typeof value !== "number") {
+      throw new Error("non-number passed to setIntPref");
+    }
+    let thePref = this._findOrCreatePref(prefName, value, true, value);
+    if (thePref._type !== PREF_INT) {
+      throw new Error(`${prefName} does not have int type`);
+    }
+    thePref._set(value);
+  },
+
+  /** @see nsIPrefBranch.clearUserPref */
+  clearUserPref: function(prefName) {
+    let thePref = this._findPref(prefName);
+    thePref._clearUserValue();
+  },
+
+  /** @see nsIPrefBranch.prefHasUserValue */
+  prefHasUserValue: function(prefName) {
+    let thePref = this._findPref(prefName);
+    return thePref._hasUserValue;
+  },
+
+  /** @see nsIPrefBranch.addObserver */
+  addObserver: function(domain, observer, holdWeak) {
+    if (holdWeak) {
+      throw new Error("shim prefs only supports strong observers");
+    }
+
+    if (!(domain in this._observers)) {
+      this._observers[domain] = [];
+    }
+    this._observers[domain].push(observer);
+  },
+
+  /** @see nsIPrefBranch.removeObserver */
+  removeObserver: function(domain, observer) {
+    if (!(domain in this._observers)) {
+      return;
+    }
+    let index = this._observers[domain].indexOf(observer);
+    if (index >= 0) {
+      this._observers[domain].splice(index, 1);
+    }
+  },
+
+  /** @see nsIPrefService.savePrefFile */
+  savePrefFile: function(file) {
+    if (file) {
+      throw new Error("shim prefs only supports null file in savePrefFile");
+    }
+    // Nothing to do - this implementation always writes back.
+  },
+
+  /** @see nsIPrefService.getBranch */
+  getBranch: function(prefRoot) {
+    if (!prefRoot) {
+      return this;
+    }
+    if (prefRoot.endsWith(".")) {
+      prefRoot = prefRoot.slice(0, -1);
+    }
+    // This is a bit weird since it could erroneously return a pref,
+    // not a pref branch.
+    return this._findPref(prefRoot);
+  },
+
+  /**
+   * Return this preference's current value.
+   *
+   * @return {Any} The current value of this preference.  This may
+   *         return a string, a number, or a boolean depending on the
+   *         preference's type.
+   */
+  _get: function() {
+    if (this._hasUserValue) {
+      return this._userValue;
+    }
+    return this._defaultValue;
+  },
+
+  /**
+   * Set the preference's value.  The new value is assumed to be a
+   * user value.  After setting the value, this function emits a
+   * change notification.
+   *
+   * @param {Any} value the new value
+   */
+  _set: function(value) {
+    if (!this._hasUserValue || value !== this._userValue) {
+      this._userValue = value;
+      this._hasUserValue = true;
+      this._saveAndNotify();
+    }
+  },
+
+  /**
+   * Set the default value for this preference, and emit a
+   * notification if this results in a visible change.
+   *
+   * @param {Any} value the new default value
+   */
+  _setDefault: function(value) {
+    if (this._defaultValue !== value) {
+      this._defaultValue = value;
+      if (!this._hasUserValue) {
+        this._saveAndNotify();
+      }
+    }
+  },
+
+  /**
+   * If this preference has a user value, clear it.  If a change was
+   * made, emit a change notification.
+   */
+  _clearUserValue: function() {
+    if (this._hasUserValue) {
+      this._userValue = null;
+      this._hasUserValue = false;
+      this._saveAndNotify();
+    }
+  },
+
+  /**
+   * Helper function to write the preference's value to local storage
+   * and then emit a change notification.
+   */
+  _saveAndNotify: function() {
+    let store = {
+      type: this._type,
+      defaultValue: this._defaultValue,
+      hasUserValue: this._hasUserValue,
+      userValue: this._userValue,
+    };
+
+    localStorage.setItem(PREFIX + this._fullName, JSON.stringify(store));
+    this._parent._notify(this._name);
+  },
+
+  /**
+   * Change this preference's value without writing it back to local
+   * storage.  This is used to handle changes to local storage that
+   * were made externally.
+   *
+   * @param {Number} type one of the PREF_* values
+   * @param {Any} userValue the user value to use if the pref does not exist
+   * @param {Any} defaultValue the default value to use if the pref
+   *        does not exist
+   * @param {Boolean} hasUserValue if a new pref is created, whether
+   *        the default value is also a user value
+   * @param {Object} store the new value of the preference.  It should
+   *        be of the form {type, defaultValue, hasUserValue, userValue};
+   *        where |type| is one of the PREF_* type constants; |defaultValue|
+   *        and |userValue| are the default and user values, respectively;
+   *        and |hasUserValue| is a boolean indicating whether the user value
+   *        is valid
+   */
+  _storageUpdated: function(type, userValue, hasUserValue, defaultValue) {
+    this._type = type;
+    this._defaultValue = defaultValue;
+    this._hasUserValue = hasUserValue;
+    this._userValue = userValue;
+    // There's no need to write this back to local storage, since it
+    // came from there; and this avoids infinite event loops.
+    this._parent._notify(this._name);
+  },
+
+  /**
+   * Helper function to find either a Preference or PrefBranch object
+   * given its name.  If the name is not found, throws an exception.
+   *
+   * @param {String} prefName the fully-qualified preference name
+   * @return {Object} Either a Preference or PrefBranch object
+   */
+  _findPref: function(prefName) {
+    let branchNames = prefName.split(".");
+    let branch = this;
+
+    for (let branchName of branchNames) {
+      branch = branch._children[branchName];
+      if (!branch) {
+        // throw new Error(`could not find pref branch ${ prefName}`);
+        return false;
+
+      }
+    }
+
+    return branch;
+  },
+
+  /**
+   * Helper function to notify any observers when a preference has
+   * changed.  This will also notify the parent branch for further
+   * reporting.
+   *
+   * @param {String} relativeName the name of the updated pref,
+   *        relative to this branch
+   */
+  _notify: function(relativeName) {
+    for (let domain in this._observers) {
+      if (relativeName === domain || domain === "" ||
+          (domain.endsWith(".") && relativeName.startsWith(domain))) {
+        // Allow mutation while walking.
+        let localList = this._observers[domain].slice();
+        for (let observer of localList) {
+          try {
+            if ("observe" in observer) {
+              observer.observe(this, NS_PREFBRANCH_PREFCHANGE_TOPIC_ID,
+                               relativeName);
+            } else {
+              // Function-style observer -- these aren't mentioned in
+              // the IDL, but they're accepted and devtools uses them.
+              observer(this, NS_PREFBRANCH_PREFCHANGE_TOPIC_ID, relativeName);
+            }
+          } catch (e) {
+            console.error(e);
+          }
+        }
+      }
+    }
+
+    if (this._parent) {
+      this._parent._notify(`${this._name }.${ relativeName}`);
+    }
+  },
+
+  /**
+   * Helper function to create a branch given an array of branch names
+   * representing the path of the new branch.
+   *
+   * @param {Array} branchList an array of strings, one per component
+   *        of the branch to be created
+   * @return {PrefBranch} the new branch
+   */
+  _createBranch: function(branchList) {
+    let parent = this;
+    for (let branch of branchList) {
+      if (!parent._children[branch]) {
+        let isParentRoot = !parent._parent;
+        let branchName = (isParentRoot ? "" : `${parent.root }.`) + branch;
+        parent._children[branch] = new PrefBranch(parent, branch, branchName);
+      }
+      parent = parent._children[branch];
+    }
+    return parent;
+  },
+
+  /**
+   * Create a new preference.  The new preference is assumed to be in
+   * local storage already, and the new value is taken from there.
+   *
+   * @param {String} keyName the full-qualified name of the preference.
+   *        This is also the name of the key in local storage.
+   * @param {Any} userValue the user value to use if the pref does not exist
+   * @param {Boolean} hasUserValue if a new pref is created, whether
+   *        the default value is also a user value
+   * @param {Any} defaultValue the default value to use if the pref
+   *        does not exist
+   * @param {Boolean} init if true, then this call is initialization
+   *        from local storage and should override the default prefs
+   */
+  _findOrCreatePref: function(keyName, userValue, hasUserValue, defaultValue,
+                               init = false) {
+    let branch = this._createBranch(keyName.split("."));
+
+    if (hasUserValue && typeof (userValue) !== typeof (defaultValue)) {
+      throw new Error(`inconsistent values when creating ${ keyName}`);
+    }
+
+    let type;
+    switch (typeof (defaultValue)) {
+      case "boolean":
+        type = PREF_BOOL;
+        break;
+      case "number":
+        type = PREF_INT;
+        break;
+      case "string":
+        type = PREF_STRING;
+        break;
+      default:
+        throw new Error(`unhandled argument type: ${ typeof (defaultValue)}`);
+    }
+
+    if (init || branch._type === PREF_INVALID) {
+      branch._storageUpdated(type, userValue, hasUserValue, defaultValue);
+    } else if (branch._type !== type) {
+      throw new Error(`attempt to change type of pref ${ keyName}`);
+    }
+
+    return branch;
+  },
+
+  getKeyName: function(keyName) {
+    if (keyName.startsWith(PREFIX)) {
+      return keyName.slice(PREFIX.length);
+    }
+
+    return keyName;
+  },
+
+  /**
+   * Helper function that is called when local storage changes.  This
+   * updates the preferences and notifies pref observers as needed.
+   *
+   * @param {StorageEvent} event the event representing the local
+   *        storage change
+   */
+  _onStorageChange: function(event) {
+    if (event.storageArea !== localStorage) {
+      return;
+    }
+
+    const key = this.getKeyName(event.key);
+
+    // Ignore delete events.  Not clear what's correct.
+    if (key === null || event.newValue === null) {
+      return;
+    }
+
+    let { type, userValue, hasUserValue, defaultValue } =
+        JSON.parse(event.newValue);
+    if (event.oldValue === null) {
+      this._findOrCreatePref(key, userValue, hasUserValue, defaultValue);
+    } else {
+      let thePref = this._findPref(key);
+      thePref._storageUpdated(type, userValue, hasUserValue, defaultValue);
+    }
+  },
+
+  /**
+   * Helper function to initialize the root PrefBranch.
+   */
+  _initializeRoot: function() {
+    if (Services._defaultPrefsEnabled) {
+      /* eslint-disable no-eval */
+      // let devtools = require("raw!prefs!devtools/client/preferences/devtools");
+      // eval(devtools);
+      // let all = require("raw!prefs!modules/libpref/init/all");
+      // eval(all);
+      /* eslint-enable no-eval */
+    }
+
+    // Read the prefs from local storage and create the local
+    // representations.
+    for (let i = 0; i < localStorage.length; ++i) {
+      let keyName = localStorage.key(i);
+      if (keyName.startsWith(PREFIX)) {
+        let { userValue, hasUserValue, defaultValue } =
+            JSON.parse(localStorage.getItem(keyName));
+        this._findOrCreatePref(keyName.slice(PREFIX.length), userValue,
+                               hasUserValue, defaultValue, true);
+      }
+    }
+
+    this._onStorageChange = this._onStorageChange.bind(this);
+    window.addEventListener("storage", this._onStorageChange);
+  },
+};
+
+
+const Services = {
+  _prefs: null,
+
+  _defaultPrefsEnabled: true,
+
+  get prefs() {
+    if (!this._prefs) {
+      this._prefs = new PrefBranch(null, "", "");
+      this._prefs._initializeRoot();
+    }
+    return this._prefs;
+  },
+
   appinfo: "",
-  prefs: { getBoolPref: () => {}, addObserver: () => {} },
-  obs: { addObserver: () => {} },
+  obs: { addObserver: () => {} }
 };
+
+function pref(name, value) {
+  let thePref = Services.prefs._findOrCreatePref(name, value, true, value);
+  thePref._setDefault(value);
+}
+
+module.exports = Services;
+Services.pref = pref;

--- a/devtools/client/debugger/src/utils/prefs.js
+++ b/devtools/client/debugger/src/utils/prefs.js
@@ -1,15 +1,16 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at <http://mozilla.org/MPL/2.0/>. */
 
 // @flow
 
-import { PrefsHelper } from "devtools-modules";
+// $FlowIgnore
+const { PrefsHelper } = require("devtools/client/shared/prefs");
 
 import { isDevelopment } from "devtools-environment";
 import Services from "devtools-services";
 
 // Schema version to bump when the async store format has changed incompatibly
 // and old stores should be cleared.
 const prefsSchemaVersion = 11;
 const { pref } = Services;

--- a/devtools/client/shared/prefs.js
+++ b/devtools/client/shared/prefs.js
@@ -33,41 +33,51 @@ const EventEmitter = require("devtools/s
  *        An object containing { accessorName: [prefType, prefName] } keys.
  */
 function PrefsHelper(prefsRoot = "", prefsBlueprint = {}) {
   EventEmitter.decorate(this);
 
   const cache = new Map();
 
   for (const accessorName in prefsBlueprint) {
-    const [prefType, prefName] = prefsBlueprint[accessorName];
-    map(this, cache, accessorName, prefType, prefsRoot, prefName);
+    const [prefType, prefName, fallbackValue] = prefsBlueprint[accessorName];
+    map(
+      this,
+      cache,
+      accessorName,
+      prefType,
+      prefsRoot,
+      prefName,
+      fallbackValue
+    );
   }
 
   const observer = makeObserver(this, cache, prefsRoot, prefsBlueprint);
   this.registerObserver = () => observer.register();
   this.unregisterObserver = () => observer.unregister();
 }
 
 /**
  * Helper method for getting a pref value.
  *
  * @param Map cache
  * @param string prefType
  * @param string prefsRoot
  * @param string prefName
+ * @param string|int|boolean fallbackValue
  * @return any
  */
-function get(cache, prefType, prefsRoot, prefName) {
+function get(cache, prefType, prefsRoot, prefName, fallbackValue) {
   const cachedPref = cache.get(prefName);
   if (cachedPref !== undefined) {
     return cachedPref;
   }
   const value = Services.prefs["get" + prefType + "Pref"](
-    [prefsRoot, prefName].join(".")
+    [prefsRoot, prefName].join("."),
+    fallbackValue
   );
   cache.set(prefName, value);
   return value;
 }
 
 /**
  * Helper method for setting a pref value.
  *
@@ -92,50 +102,53 @@ function set(cache, prefType, prefsRoot,
  * using the standard JSON serializer).
  *
  * @param PrefsHelper self
  * @param Map cache
  * @param string accessorName
  * @param string prefType
  * @param string prefsRoot
  * @param string prefName
+ * @param string|int|boolean fallbackValue
  * @param array serializer [optional]
  */
 function map(
   self,
   cache,
   accessorName,
   prefType,
   prefsRoot,
   prefName,
+  fallbackValue,
   serializer = { in: e => e, out: e => e }
 ) {
   if (prefName in self) {
     throw new Error(
       `Can't use ${prefName} because it overrides a property` +
         "on the instance."
     );
   }
   if (prefType == "Json") {
-    map(self, cache, accessorName, "Char", prefsRoot, prefName, {
+    map(self, cache, accessorName, "Char", prefsRoot, prefName, fallbackValue, {
       in: JSON.parse,
       out: JSON.stringify,
     });
     return;
   }
   if (prefType == "Float") {
-    map(self, cache, accessorName, "Char", prefsRoot, prefName, {
+    map(self, cache, accessorName, "Char", prefsRoot, prefName, fallbackValue, {
       in: Number.parseFloat,
       out: n => n + "",
     });
     return;
   }
 
   Object.defineProperty(self, accessorName, {
-    get: () => serializer.in(get(cache, prefType, prefsRoot, prefName)),
+    get: () =>
+      serializer.in(get(cache, prefType, prefsRoot, prefName, fallbackValue)),
     set: e => set(cache, prefType, prefsRoot, prefName, serializer.out(e)),
   });
 }
 
 /**
  * Finds the accessor for the provided pref, based on the blueprint object
  * used in the constructor.
  *

--- a/devtools/shared/event-emitter.js
+++ b/devtools/shared/event-emitter.js
@@ -344,17 +344,17 @@ module.exports = EventEmitter;
 const isEventHandler = listener =>
   listener && handler in listener && typeof listener[handler] === "function";
 
 const Services = require("Services");
 const { getNthPathExcluding } = require("devtools/shared/platform/stack");
 let loggingEnabled = false;
 
 if (!isWorker) {
-  loggingEnabled = Services.prefs.getBoolPref("devtools.dump.emit");
+  loggingEnabled = Services.prefs.getBoolPref("devtools.dump.emit", false);
   const observer = {
     observe: () => {
       loggingEnabled = Services.prefs.getBoolPref("devtools.dump.emit");
     },
   };
   Services.prefs.addObserver("devtools.dump.emit", observer);
 
   // Also listen for Loader unload to unregister the pref observer and