Fix some minor warnings from the analyzer in the framework.

sky/sdk/lib/framework/fn2.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 library fn;
 
 import 'app.dart';
 import 'dart:async';
 import 'dart:collection';
 import 'dart:mirrors';
@@ skipping 445 matching lines @@
     List<UINode> children
   }) : this.children = children == null ? _emptyList : children,
   super(
     key: key
   ) {
     assert(!_debugHasDuplicateIds());
   }
 
   void insert(RenderObjectWrapper child, dynamic slot) {
     assert(slot == null || slot is RenderObject);
+    assert(root is ContainerRenderObjectMixin);
     root.add(child.root, before: slot);
   }
 
   void removeChild(UINode node) {
     assert(root is ContainerRenderObjectMixin);
     root.remove(node.root);
     super.removeChild(node);
   }
 
   void _remove() {
@@ skipping 165 matching lines @@
   RenderParagraph createNode() => new RenderParagraph(text: text);
 
   final String text;
 
   Paragraph({ Object key, this.text }) : super(key: key);
 
   void syncRenderObject(UINode old) {
     super.syncRenderObject(old);
     root.text = text;
   }
+
+  void insert(RenderObjectWrapper child, dynamic slot) {
+    assert(false);
+    // Paragraph does not support having children currently
+  }
 }
 
 class FlexContainer extends MultiChildRenderObjectWrapper {
   RenderFlex root;
   RenderFlex createNode() => new RenderFlex(direction: this.direction);
 
   final FlexDirection direction;
 
   FlexContainer({
     Object key,
@@ skipping 23 matching lines @@
     Object key,
     this.src,
     this.size
   }) : super(key: key);
 
   void syncRenderObject(UINode old) {
     super.syncRenderObject(old);
     root.src = src;
     root.requestedSize = size;
   }
+
+  void insert(RenderObjectWrapper child, dynamic slot) {
+    assert(false);
+    // Image does not support having children currently
+  }
 }
 
 
 Set<Component> _mountedComponents = new HashSet<Component>();
 Set<Component> _unmountedComponents = new HashSet<Component>();
 
 void _enqueueDidMount(Component c) {
   assert(!_notifingMountStatus);
   _mountedComponents.add(c);
 }
@@ skipping 280 matching lines @@
     assert(root.parent is RenderView);
   }
 }
 
 class Text extends Component {
   Text(this.data) : super(key: '*text*');
   final String data;
   bool get interchangeable => true;
   UINode build() => new Paragraph(text: data);
 }