Fix some minor warnings from the analyzer in the framework.

sky/sdk/lib/framework/fn2.dart

Index: sky/sdk/lib/framework/fn2.dart
diff --git a/sky/sdk/lib/framework/fn2.dart b/sky/sdk/lib/framework/fn2.dart
index 14c41e38783487ebb657a3e85b8df7c8b180fcda..fc9770624621cce7a70e2df720b7384c24c95299 100644
--- a/sky/sdk/lib/framework/fn2.dart
+++ b/sky/sdk/lib/framework/fn2.dart
@@ -463,6 +463,7 @@ abstract class MultiChildRenderObjectWrapper extends RenderObjectWrapper {
 
   void insert(RenderObjectWrapper child, dynamic slot) {
     assert(slot == null || slot is RenderObject);
+    assert(root is ContainerRenderObjectMixin);
     root.add(child.root, before: slot);
   }
 
@@ -648,6 +649,11 @@ class Paragraph extends RenderObjectWrapper {
     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 {
@@ -691,6 +697,11 @@ class Image extends RenderObjectWrapper {
     root.src = src;
     root.requestedSize = size;
   }
+
+  void insert(RenderObjectWrapper child, dynamic slot) {
+    assert(false);
+    // Image does not support having children currently
+  }
 }