unfork DDC's copy of most SDK libraries

pkg/dev_compiler/tool/input_sdk/private/js_helper.dart

 // Copyright (c) 2013, 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 dart._js_helper;
 
 import 'dart:collection';
 
 import 'dart:_foreign_helper' show
     JS,
     JS_STRING_CONCAT;
 
 import 'dart:_interceptors';
 import 'dart:_internal' show
-    EfficientLength,
+    EfficientLengthIterable,
     MappedIterable,
     IterableElementError;
 
 import 'dart:_native_typed_data';
 
 part 'annotations.dart';
 part 'linked_hash_map.dart';
 part 'native_helper.dart';
 part 'regexp_helper.dart';
 part 'string_helper.dart';
@@ skipping 879 matching lines @@
   // we have no way of telling the compiler yet, so it will generate an extra
   // layer of indirection that wraps the SyncIterator.
   _jsIterator() => JS('', '#(...#)', _generator, _args);
 
   Iterator<E> get iterator => new SyncIterator<E>(_jsIterator());
 }
 
 class BooleanConversionAssertionError extends AssertionError {
   toString() => 'Failed assertion: boolean expression must not be null';
 }