Remove workarounds that avoided 'super' in mixins

Dart supports this properly now.

packages/flutter/lib/src/rendering/box.dart

@@ -631,7 +631,8 @@ abstract class RenderBox extends RenderObject {
   String debugDescribeSettings(String prefix) => '${super.debugDescribeSettings(prefix)}${prefix}size: ${size}\n';
 }
 
-/// A mixin that provides useful default behaviors for boxes with children managed by the [ContainerRenderObjectMixin] mixin
+/// A mixin that provides useful default behaviors for boxes with children
+/// managed by the [ContainerRenderObjectMixin] mixin.
 ///
 /// By convention, this class doesn't override any members of the superclass.
 /// Instead, it provides helpful functions that subclasses can call as

packages/flutter/lib/src/rendering/node.dart

@@ -68,32 +68,20 @@ class AbstractNode {
 
   /// Mark this node as attached.
   ///
-  /// Typically called only from overrides of [attachChildren] and to mark the
-  /// root of a tree attached.
+  /// Typically called only from the parent's attach(), and to mark the root of
+  /// a tree attached.
   void attach() {
     _attached = true;
-    attachChildren();
   }
 
-  /// Override this function in subclasses with child to call attach() for each
-  /// child. Do not call directly.
-  attachChildren() { }
-
   /// Mark this node as detached.
   ///
-  /// Typically called only from overrides for [detachChildren] and to mark the
-  /// root of a tree detached.
+  /// Typically called only from the parent's detach(), and to mark the root of
+  /// a tree detached.
   void detach() {
     _attached = false;
-    detachChildren();
   }
 
-  /// Override this function in subclasses with child to call detach() for each
-  /// child. Do not call directly.
-  detachChildren() { }
-
-  // TODO(ianh): remove attachChildren()/detachChildren() workaround once mixins can use super.
-
   AbstractNode _parent;
   /// The parent of this node in the tree.
   AbstractNode get parent => _parent;

packages/flutter/lib/src/rendering/object.dart

@@ -25,15 +25,15 @@ typedef sky.Shader ShaderCallback(Rect bounds);
 /// input parameters to the parent's layout algorithm or their position relative
 /// to other children.
 class ParentData {
-  void detach() {
-    detachSiblings();
-  }
-  void detachSiblings() { } // workaround for lack of inter-class mixins in Dart
+  /// Called when the RenderObject is removed from the tree.
+  void detach() { }
 
-  /// Override this function in subclasses to merge in data from other instance into this instance
+  /// Override this function in subclasses to merge in data from other instance
+  /// into this instance.
   void merge(ParentData other) {
     assert(other.runtimeType == this.runtimeType);
   }
+
   String toString() => '<none>';
 }
 
@@ -1158,11 +1158,13 @@ abstract class RenderObjectWithChildMixin<ChildType extends RenderObject> implem
     if (_child != null)
       adoptChild(_child);
   }
-  void attachChildren() {
+  void attach() {
+    super.attach();
     if (_child != null)
       _child.attach();
   }
-  void detachChildren() {
+  void detach() {
+    super.detach();
     if (_child != null)
       _child.detach();
   }
@@ -1178,14 +1180,15 @@ abstract class RenderObjectWithChildMixin<ChildType extends RenderObject> implem
 }
 
 /// Parent data to support a doubly-linked list of children
-abstract class ContainerParentDataMixin<ChildType extends RenderObject> {
+abstract class ContainerParentDataMixin<ChildType extends RenderObject> implements ParentData {
   /// The previous sibling in the parent's child list
   ChildType previousSibling;
   /// The next sibling in the parent's child list
   ChildType nextSibling;
 
   /// Clear the sibling pointers.
-  void detachSiblings() {
+  void detach() {
+    super.detach();
     if (previousSibling != null) {
       assert(previousSibling.parentData is ContainerParentDataMixin<ChildType>);
       assert(previousSibling != this);
@@ -1362,6 +1365,27 @@ abstract class ContainerRenderObjectMixin<ChildType extends RenderObject, Parent
     _removeFromChildList(child);
     _addToChildList(child, before: before);
   }
+
+  void attach() {
+    super.attach();
+    ChildType child = _firstChild;
+    while (child != null) {
+      child.attach();
+      assert(child.parentData is ParentDataType);
+      child = child.parentData.nextSibling;
+    }
+  }
+
+  void detach() {
+    super.detach();
+    ChildType child = _firstChild;
+    while (child != null) {
+      child.detach();
+      assert(child.parentData is ParentDataType);
+      child = child.parentData.nextSibling;
+    }
+  }
+
   void redepthChildren() {
     ChildType child = _firstChild;
     while (child != null) {
@@ -1370,22 +1394,7 @@ abstract class ContainerRenderObjectMixin<ChildType extends RenderObject, Parent
       child = child.parentData.nextSibling;
     }
   }
-  void attachChildren() {
-    ChildType child = _firstChild;
-    while (child != null) {
-      child.attach();
-      assert(child.parentData is ParentDataType);
-      child = child.parentData.nextSibling;
-    }
-  }
-  void detachChildren() {
-    ChildType child = _firstChild;
-    while (child != null) {
-      child.detach();
-      assert(child.parentData is ParentDataType);
-      child = child.parentData.nextSibling;
-    }
-  }
+
   void visitChildren(RenderObjectVisitor visitor) {
     ChildType child = _firstChild;
     while (child != null) {