Add a type on lazyAsJsDate

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,
@@ skipping 382 matching lines @@
     var date = JS('', r'new Date(#)', value);
     if (isUtc) {
       JS('num', r'#.setUTCFullYear(#)', date, years);
     } else {
       JS('num', r'#.setFullYear(#)', date, years);
     }
     return JS('num', r'#.valueOf()', date);
   }
 
   // Lazily keep a JS Date stored in the JS object.
-  static lazyAsJsDate(receiver) {
+  static lazyAsJsDate(DateTime receiver) {
     if (JS('bool', r'#.date === (void 0)', receiver)) {
       JS('void', r'#.date = new Date(#)', receiver,
          receiver.millisecondsSinceEpoch);
     }
     return JS('var', r'#.date', receiver);
   }
 
   // The getters for date and time parts below add a positive integer to ensure
   // that the result is really an integer, because the JavaScript implementation
   // may return -0.0 instead of 0.
@@ skipping 450 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';
 }