fixes browser/runtime_test, and a few extension member fixes

lib/runtime/dart/convert.js

Index: lib/runtime/dart/convert.js
diff --git a/lib/runtime/dart/convert.js b/lib/runtime/dart/convert.js
index 95e6904ca1471e2ff8156a00c02e48782858861f..d59e379d1aa38576012b9a83d4590c2d1fe5110e 100644
--- a/lib/runtime/dart/convert.js
+++ b/lib/runtime/dart/convert.js
@@ -2838,6 +2838,7 @@ var collection = dart.import(collection);
     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;