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

lib/compiler/implementation/dart_backend/renamer.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.
 
 /**
  * Renames only top-level elements that would let to ambiguity if not renamed.
  * TODO(smok): Make sure that top-level fields are correctly renamed.
  */
 void renamePlaceholders(
     Compiler compiler,
@@ skipping 22 matching lines @@
 
   String renameElement(Element element) {
     assert(Elements.isStaticOrTopLevel(element)
            || element is TypeVariableElement);
     // TODO(smok): Make sure that the new name does not conflict with existing
     // local identifiers.
     // TODO(smok): We may want to reuse class static field and method names.
     String originalName = element.name.slowToString();
     LibraryElement library = element.getLibrary();
     if (isDartCoreLib(compiler, library)) {
+      assert(element.isTopLevel());
       final prefix =
           imports.putIfAbsent(library, () => generateUniqueName('p'));
       return '$prefix.$originalName';
     }
 
     return rename(library, originalName);
   }
 
   renameNodes(Collection<Node> nodes, renamer) {
     final comparison = compareBy((node) => node.getBeginToken().charOffset);
@@ skipping 67 matching lines @@
     int length = alphabet.length;
     StringBuffer resultBuilder = new StringBuffer();
     while (index >= length) {
       resultBuilder.add(alphabet[index % length]);
       index ~/= length;
     }
     resultBuilder.add(alphabet[index]);
     return resultBuilder.toString();
   }
 }