Support async/await in the dart2js frontend.

sdk/lib/_internal/compiler/implementation/elements/modelx.dart

Index: sdk/lib/_internal/compiler/implementation/elements/modelx.dart
diff --git a/sdk/lib/_internal/compiler/implementation/elements/modelx.dart b/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
index 3ddd8295711c0cdeee46b1190112c4736c802c95..d435f4aa9c96e0acd7784ba785f71ee035330c3b 100644
--- a/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
+++ b/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
@@ -287,6 +287,7 @@ class ErroneousElementX extends ElementX implements ErroneousElement {
     throw 'unsupported operation on erroneous element';
   }
 
+  get asyncMarker => AsyncMarker.SYNC;
   Link<MetadataAnnotation> get metadata => unsupported();
   bool get hasNode => false;
   get node => unsupported();
@@ -1540,6 +1541,8 @@ abstract class BaseFunctionElementX
 
   AbstractFieldElement abstractField;
 
+  AsyncMarker asyncMarker = AsyncMarker.SYNC;
+
   BaseFunctionElementX(String name,
                        ElementKind kind,
                        Modifiers this.modifiers,