fixes for angular hello: String + and disable broken arg parsing

lib/runtime/dart/async.js

Index: lib/runtime/dart/async.js
diff --git a/lib/runtime/dart/async.js b/lib/runtime/dart/async.js
index 2c1e2bdb32a9497bd64deea4b17736928a6da52e..5f3d501eea0ca15345fc305119c1a6e8b8c944eb 100644
--- a/lib/runtime/dart/async.js
+++ b/lib/runtime/dart/async.js
@@ -43,7 +43,7 @@ var async;
     toString() {
       let result = `Uncaught Error: ${this.error}`;
       if (this.stackTrace != null) {
-        result = core.String['+'](result, `\nStack Trace:\n${this.stackTrace}`);
+        result = dart.notNull(result) + `\nStack Trace:\n${this.stackTrace}`;
       }
       return result;
     }