Preserve async markers when parsing js_ast

pkg/dev_compiler/lib/src/js_ast/template.dart

Index: pkg/dev_compiler/lib/src/js_ast/template.dart
diff --git a/pkg/dev_compiler/lib/src/js_ast/template.dart b/pkg/dev_compiler/lib/src/js_ast/template.dart
index efac3f4e3544ab4a789adf240ec714e13d7ad402..20fb6b62a7130d3361f6bbef911cc223c5952d97 100644
--- a/pkg/dev_compiler/lib/src/js_ast/template.dart
+++ b/pkg/dev_compiler/lib/src/js_ast/template.dart
@@ -670,7 +670,8 @@ class InstantiatorGeneratorVisitor implements NodeVisitor<Instantiator> {
       if (node is ArrowFun) {
         return new ArrowFun(params, body);
       } else if (node is Fun) {
-        return new Fun(params, body);
+        return new Fun(params, body,
+            isGenerator: node.isGenerator, asyncModifier: node.asyncModifier);
       } else {
         throw "Unknown FunctionExpression type ${node.runtimeType}: $node";
       }