Allow "const ... native" constructors and "const factory ... native" constructors in dartc

frog/lib/date_implementation.dart

 // Copyright (c) 2011, 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.
 
 // TODO(jmesserly): the native class should be the real JS Date.
 // TODO(jimhug): Making the date value non-lazy might be a good path there.
 class DateImplementation implements Date {
   final int value;
   final TimeZoneImplementation timeZone;
 
@@ skipping 246 matching lines @@
   return value;''';
 
   static int _valueFromString(String str) native
   '''var value = Date.parse(str);
   if (isNaN(value)) throw Error("Invalid Date");
   return value;''';
 
   static int _now() native "return new Date().valueOf();";
 
   // Lazily keep a JS Date stored in the dart object.
-  var _asJs() native '''
+  _asJs() native '''
   if (!this.date) {
     this.date = new Date(this.value);
   }
   return this.date;''';
 }
 
 // Trivial implementation of TimeZone
 class TimeZoneImplementation implements TimeZone {
   const TimeZoneImplementation.utc() : this.isUtc = true;
   const TimeZoneImplementation.local() : this.isUtc = false;
 
   bool operator ==(other) {
     if (!(other is TimeZoneImplementation)) return false;
     return isUtc == other.isUtc;
   }
 
   String toString() {
     if (isUtc) return "TimeZone (UTC)";
     return "TimeZone (Local)";
   }
 
   final bool isUtc;
 }