Make EfficientLength public, as EfficientLengthIterable.

sdk/lib/_internal/compiler/js_lib/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 _js_helper;
 
 import 'dart:_async_await_error_codes' as async_error_codes;
 
 import 'dart:_js_embedded_names' show
     DEFERRED_LIBRARY_URIS,
@@ skipping 51 matching lines @@
     JS_NULL_CLASS_NAME,
     JS_OBJECT_CLASS_NAME,
     JS_OPERATOR_AS_PREFIX,
     JS_OPERATOR_IS_PREFIX,
     JS_SIGNATURE_NAME,
     JS_STRING_CONCAT,
     RAW_DART_FUNCTION_REF;
 
 import 'dart:_interceptors';
 import 'dart:_internal' as _symbol_dev;
-import 'dart:_internal' show EfficientLength, MappedIterable;
+import 'dart:_internal' show MappedIterable;
 
 import 'dart:_native_typed_data';
 
 import 'dart:_js_names' show
     extractKeys,
     mangledNames,
     unmangleGlobalNameIfPreservedAnyways,
     unmangleAllIdentifiersIfPreservedAnyways;
 
 part 'annotations.dart';
@@ skipping 3962 matching lines @@
   // This is a function that will return a helper function that does the
   // iteration of the sync*.
   //
   // Each invocation should give a body with fresh state.
   final dynamic /* js function */ _outerHelper;
 
   SyncStarIterable(this._outerHelper);
 
   Iterator get iterator => new SyncStarIterator(JS('', '#()', _outerHelper));
 }