Replace dart_core.js with actual compiled SDK

test/generated_sdk/lib/core/core.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.
 
 /**
  *
  * Built-in types, collections,
  * and other core functionality for every Dart program.
  *
  * This library is automatically imported.
@@ skipping 183 matching lines @@
 import "dart:_internal" as _symbol_dev;
 import 'dart:_interceptors';
 import 'dart:_js_helper' show patch,
                               checkInt,
                               getRuntimeType,
                               jsonEncodeNative,
                               JSSyntaxRegExp,
                               Primitives,
                               stringJoinUnchecked,
                               objectHashCode;
+import 'dart:_foreign_helper' show JS;
 
 String _symbolToString(Symbol symbol) => _symbol_dev.Symbol.getName(symbol);
 _symbolMapToStringMap(Map<Symbol, dynamic> map) {
   if (map == null) return null;
   var result = new Map<String, dynamic>();
   map.forEach((Symbol key, value) {
     result[_symbolToString(key)] = value;
   });
   return result;
 }
-class _ListConstructorSentinel extends JSInt {
+class _ListConstructorSentinel {
   const _ListConstructorSentinel();
 }