fixes browser/runtime_test, and a few extension member fixes

lib/runtime/dart/convert.js

 var convert = dart.defineLibrary(convert, {});
 var core = dart.import(core);
 var async = dart.import(async);
 var typed_data = dart.import(typed_data);
 var _internal = dart.import(_internal);
 var collection = dart.import(collection);
 (function(exports, core, async, typed_data, _internal, collection) {
   'use strict';
   let Codec$ = dart.generic(function(S, T) {
     class Codec extends core.Object {
@@ skipping 2820 matching lines @@
       return dart.notNull(this[_parent][_isUpgraded]) ? this[_parent].keys[dartx.iterator] : this[_parent][_computeKeys]()[dartx.iterator];
     }
     contains(key) {
       return this[_parent].containsKey(key);
     }
   }
   dart.setSignature(_JsonMapKeyIterable, {
     constructors: () => ({_JsonMapKeyIterable: [_JsonMapKeyIterable, [_JsonMap]]}),
     methods: () => ({elementAt: [core.String, [core.int]]})
   });
+  dart.defineExtensionMembers(_JsonMapKeyIterable, ['elementAt', 'contains', 'length', 'iterator']);
   class _JsonDecoderSink extends _StringSinkConversionSink {
     _JsonDecoderSink(reviver, sink) {
       this[_reviver] = reviver;
       this[_sink] = sink;
       super._StringSinkConversionSink(new core.StringBuffer());
     }
     close() {
       super.close();
       let buffer = dart.as(this[_stringSink], core.StringBuffer);
       let accumulated = dart.toString(buffer);
@@ skipping 39 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;
 })(convert, core, async, typed_data, _internal, collection);