Normalize type masks to use the least upper instantiated subclass/type.

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;
 
 const DEFAULT_PLATFORM_CONFIG = """
 [libraries]
@@ skipping 45 matching lines @@
   'int': 'abstract class int extends num { }',
   'Iterable': '''
       abstract class Iterable<E> {
           Iterator<E> get iterator => null;
       }''',
   'Iterator': '''
       abstract class Iterator<E> {
           E get current => null;
       }''',
   'LinkedHashMap': r'''
-      class LinkedHashMap {
+      class LinkedHashMap<K, V> implements Map<K, V> {
         factory LinkedHashMap._empty() => null;
         factory LinkedHashMap._literal(elements) => null;
         static _makeEmpty() => null;
         static _makeLiteral(elements) => null;
       }''',
   'List': r'''
       class List<E> extends Iterable<E> {
         var length;
         List([length]);
         List.filled(length, element);
@@ skipping 379 matching lines @@
 
     const LookupMap(this._entries, [this._nestedMaps = const []])
         : _key = null, _value = null;
 
     const LookupMap.pair(this._key, this._value)
         : _entries = const [], _nestedMaps = const [];
     V operator[](K k) => null;
   }''',
   '_version': 'const _version = "0.0.1+1";',
 };