new file mode 100644
--- /dev/null
+++ b/editor/AsyncSpellCheckTestHelper.jsm
@@ -0,0 +1,86 @@
+/* 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/. */
+
+this.EXPORTED_SYMBOLS = [
+  "onSpellCheck",
+];
+
+const SPELL_CHECK_ENDED_TOPIC = "inlineSpellChecker-spellCheck-ended";
+const SPELL_CHECK_STARTED_TOPIC = "inlineSpellChecker-spellCheck-started";
+
+const { classes: Cc, interfaces: Ci, utils: Cu } = Components;
+
+/**
+ * Waits until spell checking has stopped on the given element.
+ *
+ * When a spell check is pending, this waits indefinitely until the spell check
+ * ends.  When a spell check is not pending, it waits a small number of turns of
+ * the event loop: if a spell check begins, it resumes waiting indefinitely for
+ * the end, and otherwise it stops waiting and calls the callback.
+ *
+ * This this can therefore trap spell checks that have not started at the time
+ * of calling, spell checks that have already started, multiple consecutive
+ * spell checks, and the absence of spell checks altogether.
+ *
+ * @param editableElement  The element being spell checked.
+ * @param callback         Called when spell check has completed or enough turns
+ *                         of the event loop have passed to determine it has not
+ *                         started.
+ */
+function onSpellCheck(editableElement, callback) {
+  let editor = editableElement.editor;
+  if (!editor) {
+    let win = editableElement.ownerDocument.defaultView;
+    editor = win.QueryInterface(Ci.nsIInterfaceRequestor).
+                 getInterface(Ci.nsIWebNavigation).
+                 QueryInterface(Ci.nsIInterfaceRequestor).
+                 getInterface(Ci.nsIEditingSession).
+                 getEditorForWindow(win);
+  }
+  if (!editor)
+    throw new Error("Unable to find editor for element " + editableElement);
+
+  try {
+    // False is important here.  Pass false so that the inline spell checker
+    // isn't created if it doesn't already exist.
+    var isc = editor.getInlineSpellChecker(false);
+  }
+  catch (err) {
+    // getInlineSpellChecker throws if spell checking is not enabled instead of
+    // just returning null, which seems kind of lame.  (Spell checking is not
+    // enabled on Android.)  The point here is only to determine whether spell
+    // check is pending, and if getInlineSpellChecker throws, then it's not
+    // pending.
+  }
+  let waitingForEnded = isc && isc.spellCheckPending;
+  let count = 0;
+
+  function observe(subj, topic, data) {
+    if (subj != editor)
+      return;
+    count = 0;
+    let expectedTopic = waitingForEnded ? SPELL_CHECK_ENDED_TOPIC :
+                        SPELL_CHECK_STARTED_TOPIC;
+    if (topic != expectedTopic)
+      Cu.reportError("Expected " + expectedTopic + " but got " + topic + "!");
+    waitingForEnded = !waitingForEnded;
+  }
+
+  let os = Cc["@mozilla.org/observer-service;1"].
+           getService(Ci.nsIObserverService);
+  os.addObserver(observe, SPELL_CHECK_STARTED_TOPIC, false);
+  os.addObserver(observe, SPELL_CHECK_ENDED_TOPIC, false);
+
+  let timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
+  timer.init(function tick() {
+    // Wait an arbitrarily large number -- 50 -- turns of the event loop before
+    // declaring that no spell checks will start.
+    if (waitingForEnded || ++count < 50)
+      return;
+    timer.cancel();
+    os.removeObserver(observe, SPELL_CHECK_STARTED_TOPIC);
+    os.removeObserver(observe, SPELL_CHECK_ENDED_TOPIC);
+    callback();
+  }, 0, Ci.nsITimer.TYPE_REPEATING_SLACK);
+};

--- a/editor/Makefile.in
+++ b/editor/Makefile.in
@@ -6,10 +6,14 @@
 DEPTH            := @DEPTH@
 topsrcdir        := @top_srcdir@
 srcdir           := @srcdir@
 VPATH            := @srcdir@
 FAIL_ON_WARNINGS := 1
 
 include $(DEPTH)/config/autoconf.mk
 
+EXTRA_JS_MODULES = \
+  AsyncSpellCheckTestHelper.jsm \
+  $(NULL)
+
 include $(topsrcdir)/config/rules.mk