Typing fixes to eliminate casts/dcalls

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 a5ba6200032f7e8cf2e813e3793a48416f07a74a..fdccd3354fc46a1e5bd4dca5c0388f95751a2d16 100644
--- a/lib/runtime/dart/convert.js
+++ b/lib/runtime/dart/convert.js
@@ -91,7 +91,7 @@ var convert;
       }
       bind(source) {
         dart.as(source, async.Stream$(S));
-        return new async.Stream$(T).eventTransformed(source, (sink => new _ConverterStreamEventSink(this, sink)).bind(this));
+        return new (async.Stream$(T)).eventTransformed(source, (sink => new _ConverterStreamEventSink(this, sink)).bind(this));
       }
     }
     Converter[dart.implements] = () => [async.StreamTransformer$(S, T)];