--- a/dom/base/Element.h
+++ b/dom/base/Element.h
@@ -14,16 +14,17 @@
 #define mozilla_dom_Element_h__
 
 #include "mozilla/dom/FragmentOrElement.h" // for base class
 #include "nsChangeHint.h"                  // for enum
 #include "mozilla/EventStates.h"           // for member
 #include "mozilla/dom/DirectionalityUtils.h"
 #include "nsIDOMElement.h"
 #include "nsILinkHandler.h"
+#include "nsINodeList.h"
 #include "nsNodeUtils.h"
 #include "nsAttrAndChildArray.h"
 #include "mozFlushType.h"
 #include "nsDOMAttributeMap.h"
 #include "nsPresContext.h"
 #include "mozilla/CORSMode.h"
 #include "mozilla/Attributes.h"
 #include "nsIScrollableFrame.h"

--- a/dom/base/FragmentOrElement.cpp
+++ b/dom/base/FragmentOrElement.cpp
@@ -122,16 +122,17 @@
 
 #include "mozilla/CORSMode.h"
 
 #include "mozilla/dom/ShadowRoot.h"
 #include "mozilla/dom/HTMLTemplateElement.h"
 
 #include "nsStyledElement.h"
 #include "nsIContentInlines.h"
+#include "nsChildContentList.h"
 
 using namespace mozilla;
 using namespace mozilla::dom;
 
 int32_t nsIContent::sTabFocusModel = eTabFocus_any;
 bool nsIContent::sTabFocusModelAppliesToXUL = false;
 uint64_t nsMutationGuard::sGeneration = 0;
 

--- a/dom/base/moz.build
+++ b/dom/base/moz.build
@@ -48,16 +48,17 @@ EXPORTS += [
     'mozAutoDocUpdate.h',
     'mozFlushType.h',
     'nsAtomListUtils.h',
     'nsAttrAndChildArray.h',
     'nsAttrName.h',
     'nsAttrValue.h',
     'nsAttrValueInlines.h',
     'nsCaseTreatment.h',
+    'nsChildContentList.h',
     'nsContentCID.h',
     'nsContentCreatorFunctions.h',
     'nsContentList.h',
     'nsContentListDeclarations.h',
     'nsContentPermissionHelper.h',
     'nsContentPolicyUtils.h',
     'nsContentSink.h',
     'nsContentTypeParser.h',

new file mode 100644
--- /dev/null
+++ b/dom/base/nsChildContentList.h
@@ -0,0 +1,63 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/. */
+
+#ifndef nsChildContentList_h__
+#define nsChildContentList_h__
+
+#include "nsISupportsImpl.h"
+#include "nsINodeList.h"            // base class
+#include "js/TypeDecls.h"     // for Handle, Value, JSObject, JSContext
+
+class nsIContent;
+class nsINode;
+
+/**
+ * Class that implements the nsIDOMNodeList interface (a list of children of
+ * the content), by holding a reference to the content and delegating GetLength
+ * and Item to its existing child list.
+ * @see nsIDOMNodeList
+ */
+class nsChildContentList final : public nsINodeList
+{
+public:
+  explicit nsChildContentList(nsINode* aNode)
+    : mNode(aNode)
+  {
+  }
+
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_SKIPPABLE_SCRIPT_HOLDER_CLASS(nsChildContentList)
+
+  // nsWrapperCache
+  virtual JSObject* WrapObject(JSContext *cx, JS::Handle<JSObject*> aGivenProto) override;
+
+  // nsIDOMNodeList interface
+  NS_DECL_NSIDOMNODELIST
+
+  // nsINodeList interface
+  virtual int32_t IndexOf(nsIContent* aContent) override;
+  virtual nsIContent* Item(uint32_t aIndex) override;
+
+  void DropReference()
+  {
+    mNode = nullptr;
+  }
+
+  virtual nsINode* GetParentObject() override
+  {
+    return mNode;
+  }
+
+private:
+  ~nsChildContentList() {}
+
+  // The node whose children make up the list.
+  // This is a non-owning ref which is safe because it's set to nullptr by
+  // DropReference() by the node slots get destroyed.
+  nsINode* MOZ_NON_OWNING_REF mNode;
+};
+
+#endif /* nsChildContentList_h__ */

--- a/dom/base/nsINode.cpp
+++ b/dom/base/nsINode.cpp
@@ -100,20 +100,26 @@
 #include "nsWrapperCacheInlines.h"
 #include "WrapperFactory.h"
 #include "DocumentType.h"
 #include <algorithm>
 #include "nsGlobalWindow.h"
 #include "nsDOMMutationObserver.h"
 #include "GeometryUtils.h"
 #include "nsIAnimationObserver.h"
+#include "nsChildContentList.h"
 
 using namespace mozilla;
 using namespace mozilla::dom;
 
+nsINode::nsSlots::nsSlots()
+  : mWeakReference(nullptr)
+{
+}
+
 nsINode::nsSlots::~nsSlots()
 {
   if (mChildNodes) {
     mChildNodes->DropReference();
   }
 
   if (mWeakReference) {
     mWeakReference->NoticeNodeDestruction();

--- a/dom/base/nsINode.h
+++ b/dom/base/nsINode.h
@@ -7,17 +7,16 @@
 #ifndef nsINode_h___
 #define nsINode_h___
 
 #include "mozilla/Likely.h"
 #include "nsCOMPtr.h"               // for member, local
 #include "nsGkAtoms.h"              // for nsGkAtoms::baseURIProperty
 #include "nsIDOMNode.h"
 #include "mozilla/dom/NodeInfo.h"            // member (in nsCOMPtr)
-#include "nsINodeList.h"            // base class
 #include "nsIVariant.h"             // for use in GetUserData()
 #include "nsNodeInfoManager.h"      // for use in NodePrincipal()
 #include "nsPropertyTable.h"        // for typedefs
 #include "nsTObserverArray.h"       // for member
 #include "mozilla/ErrorResult.h"
 #include "mozilla/MemoryReporting.h"
 #include "mozilla/dom/EventTarget.h" // for base class
 #include "js/TypeDecls.h"     // for Handle, Value, JSObject, JSContext
@@ -39,16 +38,17 @@ class nsIAnimationObserver;
 class nsIContent;
 class nsIDocument;
 class nsIDOMElement;
 class nsIDOMNodeList;
 class nsIEditor;
 class nsIFrame;
 class nsIMutationObserver;
 class nsINode;
+class nsINodeList;
 class nsIPresShell;
 class nsIPrincipal;
 class nsIURI;
 class nsNodeSupportsWeakRefTearoff;
 class nsNodeWeakReference;
 class nsDOMMutationObserver;
 
 namespace mozilla {
@@ -229,62 +229,16 @@ public:
 private:
   // This is the value sGeneration had when the guard was constructed.
   uint64_t mStartingGeneration;
 
   // This value is incremented on every mutation, for the life of the process.
   static uint64_t sGeneration;
 };
 
-/**
- * Class that implements the nsIDOMNodeList interface (a list of children of
- * the content), by holding a reference to the content and delegating GetLength
- * and Item to its existing child list.
- * @see nsIDOMNodeList
- */
-class nsChildContentList final : public nsINodeList
-{
-public:
-  explicit nsChildContentList(nsINode* aNode)
-    : mNode(aNode)
-  {
-  }
-
-  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
-  NS_DECL_CYCLE_COLLECTION_SKIPPABLE_SCRIPT_HOLDER_CLASS(nsChildContentList)
-
-  // nsWrapperCache
-  virtual JSObject* WrapObject(JSContext *cx, JS::Handle<JSObject*> aGivenProto) override;
-
-  // nsIDOMNodeList interface
-  NS_DECL_NSIDOMNODELIST
-
-  // nsINodeList interface
-  virtual int32_t IndexOf(nsIContent* aContent) override;
-  virtual nsIContent* Item(uint32_t aIndex) override;
-
-  void DropReference()
-  {
-    mNode = nullptr;
-  }
-
-  virtual nsINode* GetParentObject() override
-  {
-    return mNode;
-  }
-
-private:
-  ~nsChildContentList() {}
-
-  // The node whose children make up the list.
-  // This is a non-owning ref which is safe because it's set to nullptr by
-  // DropReference() by the node slots get destroyed.
-  nsINode* MOZ_NON_OWNING_REF mNode;
-};
-
 // This should be used for any nsINode sub-class that has fields of its own
 // that it needs to measure;  any sub-class that doesn't use it will inherit
 // SizeOfExcludingThis from its super-class.  SizeOfIncludingThis() need not be
 // defined, it is inherited from nsINode.
 // This macro isn't actually specific to nodes, and bug 956400 will move it into MFBT.
 #define NS_DECL_SIZEOF_EXCLUDING_THIS \
   virtual size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const override;
 
@@ -1062,20 +1016,17 @@ public:
    */
   virtual nsresult Clone(mozilla::dom::NodeInfo *aNodeInfo, nsINode **aResult) const = 0;
 
   // This class can be extended by subclasses that wish to store more
   // information in the slots.
   class nsSlots
   {
   public:
-    nsSlots()
-      : mWeakReference(nullptr)
-    {
-    }
+    nsSlots();
 
     // If needed we could remove the vtable pointer this dtor causes by
     // putting a DestroySlots function on nsINode
     virtual ~nsSlots();
 
     void Traverse(nsCycleCollectionTraversalCallback &cb);
     void Unlink();
 

--- a/dom/base/nsINodeList.h
+++ b/dom/base/nsINodeList.h
@@ -4,16 +4,17 @@
  * 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/. */
 
 #ifndef nsINodeList_h___
 #define nsINodeList_h___
 
 #include "nsIDOMNodeList.h"
 #include "nsWrapperCache.h"
+#include "nsIContent.h"
 
 // IID for the nsINodeList interface
 #define NS_INODELIST_IID \
 { 0xadb5e54c, 0x6e96, 0x4102, \
  { 0x8d, 0x40, 0xe0, 0x12, 0x3d, 0xcf, 0x48, 0x7a } }
 
 class nsIContent;
 class nsINode;