Remove implicit dependencies.

dart/tests/compiler/dart2js/mock_libraries.dart

 // Copyright (c) 2014, 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.
 
 // Library for creating mock versions of platform and internal libraries.
 
 library mock_libraries;
 
 String buildLibrarySource(
     Map<String, String> elementMap,
@@ skipping 52 matching lines @@
       }''',
   'Map': 'abstract class Map<K,V> {}',
   'Null': 'class Null {}',
   'num': r'''
       abstract class num {
         operator +(_);
         operator *(_);
         operator -();
       }''',
   'print': 'print(var obj) {}',
-  'proxy': 'const proxy = 0;',
+  'proxy': 'const proxy = const _Proxy();',
+  '_Proxy': '''
+      class _Proxy {
+        const _Proxy();
+      }''',
   'Object': r'''
       class Object {
         const Object();
         operator ==(other) { return true; }
         get hashCode => throw "Object.hashCode not implemented.";
         String toString() { return null; }
         noSuchMethod(im) { throw im; }
       }''',
   'StackTrace': 'abstract class StackTrace {}',
   'String': 'class String implements Pattern {}',
@@ skipping 285 matching lines @@
   'Stream': 'class Stream<T> {}',
   'Completer': 'class Completer<T> {}',
   'StreamIterator': 'class StreamIterator<T> {}',
 };
 
 const Map<String, String> DEFAULT_MIRRORS_LIBRARY = const <String, String>{
   'Comment': 'class Comment {}',
   'MirrorSystem': 'class MirrorSystem {}',
   'MirrorsUsed': 'class MirrorsUsed {}',
 };