[dart2dart] In renamer only add import prefix for top-level elements.

lib/compiler/implementation/dart_backend/placeholder_collector.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 class LocalPlaceholder implements Hashable {
   final String identifier;
   final Set<Node> nodes;
   LocalPlaceholder(this.identifier) : nodes = new Set<Node>();
   int hashCode() => identifier.hashCode();
   String toString() =>
@@ skipping 187 matching lines @@
 
   void makeNullPlaceholder(Node node) {
     assert(node is Identifier || node is Send);
     nullNodes.add(node);
   }
 
   void makeElementPlaceholder(Node node, Element element) {
     assert(element !== null);
     if (element === entryFunction) return;
     if (element.getLibrary() === coreLibrary) return;
+    if (isDartCoreLib(compiler, element.getLibrary())
+        && !element.isTopLevel()) {
+      return;
+    }
     if (element == compiler.types.dynamicType.element) {
       internalError(
           'Should never make element placeholder for dynamic type element',
           node);
     }
     elementNodes.putIfAbsent(element, () => new Set<Node>()).add(node);
   }
 
   void makePrivateIdentifier(Identifier node) {
     assert(node !== null);
@@ skipping 189 matching lines @@
     }
     node.visitChildren(this);
   }
 
   visitTypedef(Typedef node) {
     assert(currentElement is TypedefElement);
     makeElementPlaceholder(node.name, currentElement);
     node.visitChildren(this);
   }
 }