fixes #217, Object members on extension types

lib/runtime/dart/core.js

 dart.library('dart/core', null, /* Imports */[
 ], /* Lazy imports */[
   'dart/_js_helper',
   'dart/_internal',
   'dart/collection',
   'dart/_interceptors',
   'dart/convert'
 ], function(exports, _js_helper, _internal, collection, _interceptors, convert) {
   'use strict';
   class Object {
@@ skipping 58 matching lines @@
   });
   let proxy = dart.const(new _Proxy());
   dart.defineExtensionNames([
     'toString'
   ]);
   class bool extends Object {
     static fromEnvironment(name, opts) {
       let defaultValue = opts && 'defaultValue' in opts ? opts.defaultValue : false;
       throw new UnsupportedError('bool.fromEnvironment can only be used as a const constructor');
     }
-    [dartx.toString]() {
+    toString() {
       return this ? "true" : "false";
     }
   }
   dart.setSignature(bool, {
     constructors: () => ({fromEnvironment: [bool, [String], {defaultValue: bool}]})
   });
   let Comparator$ = dart.generic(function(T) {
     let Comparator = dart.typedef('Comparator', () => dart.functionType(int, [T, T]));
     return Comparator;
   });
@@ skipping 423 matching lines @@
     }
     get inMicroseconds() {
       return this[_duration];
     }
     ['=='](other) {
       if (!dart.is(other, Duration))
         return false;
       return dart.equals(this[_duration], dart.dload(other, _duration));
     }
     get hashCode() {
-      return dart[dartx.hashCode](this[_duration]);
+      return dart.hashCode(this[_duration]);
     }
     compareTo(other) {
       return this[_duration][dartx.compareTo](other[_duration]);
     }
     toString() {
       let sixDigits = n => {
         if (dart.notNull(n) >= 100000)
           return `${n}`;
         if (dart.notNull(n) >= 10000)
           return `0${n}`;
@@ skipping 2830 matching lines @@
   exports.StackTrace = StackTrace;
   exports.Stopwatch = Stopwatch;
   exports.String = String;
   exports.RuneIterator = RuneIterator;
   exports.StringBuffer = StringBuffer;
   exports.StringSink = StringSink;
   exports.Symbol = Symbol;
   exports.Type = Type;
   exports.Uri = Uri;
 });