Start new design for interceptors and implement String.charCodeAt with it.

tests/compiler/dart2js/dart_backend_test.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.
 
 import 'dart:uri';
 import 'parser_helper.dart';
 import 'mock_compiler.dart';
 import '../../../sdk/lib/_internal/compiler/compiler.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/dart2jslib.dart' as leg;
 import '../../../sdk/lib/_internal/compiler/implementation/dart_backend/dart_backend.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/elements/elements.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/tree/tree.dart';
 
 const coreLib = r'''
 library corelib;
 class Object {}
 interface bool {}
 interface num {}
 interface int extends num {}
 interface double extends num {}
-interface String {}
+abstract class String {}
 interface Function {}
 interface List {}
 interface Map {}
 interface Closure {}
 interface Dynamic_ {}
 interface Null {}
 interface TypeError {}
 class Type {}
 class Math {
   static double parseDouble(String s) => 1.0;
@@ skipping 684 matching lines @@
   testDoubleMains();
   testStaticAccessIoLib();
   testLocalFunctionPlaceholder();
   testMinification();
   testClosureLocalsMinified();
   testParametersMinified();
   testDeclarationTypePlaceholders();
   testPlatformLibraryMemberNamesAreFixed();
   testConflictsWithCoreLib();
 }