fixes #217, Object members on extension types

lib/runtime/dart/_internal.js

 dart.library('dart/_internal', null, /* Imports */[
   'dart/core',
   'dart/collection'
 ], /* Lazy imports */[
   'dart/math',
   'dart/_interceptors',
   'dart/_js_primitives'
 ], function(exports, core, collection, math, _interceptors, _js_primitives) {
   'use strict';
   class EfficientLength extends core.Object {}
@@ skipping 2301 matching lines @@
       this[_name] = name;
     }
     validated(name) {
       this[_name] = Symbol.validatePublicSymbol(name);
     }
     ['=='](other) {
       return dart.is(other, Symbol) && this[_name] == other[_name];
     }
     get hashCode() {
       let arbitraryPrime = 664597;
-      return 536870911 & dart.notNull(arbitraryPrime) * dart.notNull(dart[dartx.hashCode](this[_name]));
+      return 536870911 & dart.notNull(arbitraryPrime) * dart.notNull(dart.hashCode(this[_name]));
     }
     toString() {
       return `Symbol("${this[_name]}")`;
     }
     static getName(symbol) {
       return symbol[_name];
     }
     static validatePublicSymbol(name) {
       if (dart.notNull(name[dartx.isEmpty]) || dart.notNull(Symbol.publicSymbolPattern.hasMatch(name)))
         return name;
@@ skipping 103 matching lines @@
   exports.ReversedListIterable = ReversedListIterable;
   exports.UnmodifiableListError = UnmodifiableListError;
   exports.NonGrowableListError = NonGrowableListError;
   exports.makeListFixedLength = makeListFixedLength;
   exports.Lists = Lists;
   exports.printToConsole = printToConsole;
   exports.Sort = Sort;
   exports.Symbol = Symbol;
   exports.POWERS_OF_TEN = POWERS_OF_TEN;
 });