Make support for async/await syntax optional in the frontend.

pkg/compiler/lib/src/common/backend_api.dart

Index: pkg/compiler/lib/src/common/backend_api.dart
diff --git a/pkg/compiler/lib/src/common/backend_api.dart b/pkg/compiler/lib/src/common/backend_api.dart
index 6b809e9967ffa50fb189e8aa751fa3d975cef1a6..df63ffaba9c974c416d71b26d2e19838a6a3c768 100644
--- a/pkg/compiler/lib/src/common/backend_api.dart
+++ b/pkg/compiler/lib/src/common/backend_api.dart
@@ -55,6 +55,9 @@ abstract class Backend {
   /// Returns true if the backend supports reflection.
   bool get supportsReflection;
 
+  /// Returns true if the backend supports reflection.
+  bool get supportsAsyncAwait;
+
   /// The [ConstantSystem] used to interpret compile-time constants for this
   /// backend.
   ConstantSystem get constantSystem;