Introduce "platform configurations" to replace categories and libraries.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;
 
+const DEFAULT_PLATFORM_CONFIG = """
+[libraries]
+core:core/core.dart
+async:async/async.dart
+_js_helper:_internal/js_runtime/lib/js_helper.dart
+_interceptors:_internal/js_runtime/lib/interceptors.dart
+_isolate_helper:_internal/js_runtime/lib/isolate_helper.dart
+""";
+
 String buildLibrarySource(
     Map<String, String> elementMap,
     [Map<String, String> additionalElementMap = const <String, String>{}]) {
   Map<String, String> map = new Map<String, String>.from(elementMap);
   if (additionalElementMap != null) {
     map.addAll(additionalElementMap);
   }
   StringBuffer sb = new StringBuffer();
   map.values.forEach((String element) {
     sb.write('$element\n');
@@ skipping 428 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";',
 };