initial sync*, part of #221

lib/runtime/dart/js.js

Index: lib/runtime/dart/js.js
diff --git a/lib/runtime/dart/js.js b/lib/runtime/dart/js.js
index a143a7bf1d8b27f7c792cc6af8556e310d1595a8..27e0b0292f0d3b71fbed385b09d194ed14372ead 100644
--- a/lib/runtime/dart/js.js
+++ b/lib/runtime/dart/js.js
@@ -41,7 +41,7 @@ dart_library.library('dart/js', null, /* Imports */[
     }
     static _convertDataTree(data) {
       let _convertedObjects = collection.HashMap.identity();
-      let _convert = o => {
+      function _convert(o) {
         if (dart.notNull(_convertedObjects.containsKey(o))) {
           return _convertedObjects.get(o);
         }
@@ -60,7 +60,7 @@ dart_library.library('dart/js', null, /* Imports */[
         } else {
           return _convertToJS(o);
         }
-      };
+      }
       dart.fn(_convert);
       return _convert(data);
     }