fixes #217, Object members on extension types

lib/runtime/dart/convert.js

 dart.library('dart/convert', null, /* Imports */[
   'dart/core',
   'dart/async',
   'dart/typed_data',
   'dart/_internal',
   'dart/collection'
 ], /* Lazy imports */[
 ], function(exports, core, async, typed_data, _internal, collection) {
   'use strict';
   let Codec$ = dart.generic(function(S, T) {
@@ skipping 1347 matching lines @@
     static printOn(object, output, toEncodable, indent) {
       let stringifier = null;
       if (indent == null) {
         stringifier = new _JsonStringStringifier(output, toEncodable);
       } else {
         stringifier = new _JsonStringStringifierPretty(output, toEncodable, indent);
       }
       dart.dsend(stringifier, 'writeObject', object);
     }
     writeNumber(number) {
-      this[_sink].write(dart[dartx.toString](number));
+      this[_sink].write(dart.toString(number));
     }
     writeString(string) {
       this[_sink].write(string);
     }
     writeStringSlice(string, start, end) {
       this[_sink].write(string[dartx.substring](start, end));
     }
     writeCharCode(charCode) {
       this[_sink].writeCharCode(charCode);
     }
@@ skipping 45 matching lines @@
       stringifier.flush();
     }
     flush() {
       if (dart.notNull(this.index) > 0) {
         dart.dcall(this.addChunk, this.buffer, 0, this.index);
       }
       this.buffer = null;
       this.index = 0;
     }
     writeNumber(number) {
-      this.writeAsciiString(dart[dartx.toString](number));
+      this.writeAsciiString(dart.toString(number));
     }
     writeAsciiString(string) {
       for (let i = 0; dart.notNull(i) < dart.notNull(string[dartx.length]); i = dart.notNull(i) + 1) {
         let char = string[dartx.codeUnitAt](i);
         dart.assert(dart.notNull(char) <= 127);
         this.writeByte(char);
       }
     }
     writeString(string) {
       this.writeStringSlice(string, 0, string[dartx.length]);
@@ skipping 1447 matching lines @@
   exports.LineSplitter = LineSplitter;
   exports.StringConversionSink = StringConversionSink;
   exports.ClosableStringSink = ClosableStringSink;
   exports.UNICODE_REPLACEMENT_CHARACTER_RUNE = UNICODE_REPLACEMENT_CHARACTER_RUNE;
   exports.UNICODE_BOM_CHARACTER_RUNE = UNICODE_BOM_CHARACTER_RUNE;
   exports.Utf8Codec = Utf8Codec;
   exports.UTF8 = UTF8;
   exports.Utf8Encoder = Utf8Encoder;
   exports.Utf8Decoder = Utf8Decoder;
 });