Support async/await in dart2dart.

pkg/compiler/lib/src/compiler.dart

Index: pkg/compiler/lib/src/compiler.dart
diff --git a/pkg/compiler/lib/src/compiler.dart b/pkg/compiler/lib/src/compiler.dart
index 9b3f9c920c7a69395e76e62a669473d6b70a014f..173f421d514a20b58b987ece1ea54dc82980d738 100644
--- a/pkg/compiler/lib/src/compiler.dart
+++ b/pkg/compiler/lib/src/compiler.dart
@@ -743,6 +743,9 @@ abstract class Compiler implements DiagnosticListener {
   /// `true` if async/await features are supported.
   final bool enableAsyncAwait;
 
+  /// `true` if the compiler uses the [JavaScriptBackend].
+  final bool emitJavaScript;
+
   /// If `true`, some values are cached for reuse in incremental compilation.
   /// Incremental compilation is basically calling [run] more than once.
   final bool hasIncrementalSupport;
@@ -1004,12 +1007,14 @@ abstract class Compiler implements DiagnosticListener {
             this.suppressWarnings: false,
             bool hasIncrementalSupport: false,
             this.enableExperimentalMirrors: false,
-            this.enableAsyncAwait: false,
+            bool enableAsyncAwait: false,
             this.allowNativeExtensions: false,
             this.generateCodeWithCompileTimeErrors: false,
             api.CompilerOutputProvider outputProvider,
             List<String> strips: const []})
-      : this.disableTypeInferenceFlag =
+      : this.emitJavaScript = emitJavaScript,
+        this.enableAsyncAwait = enableAsyncAwait || !emitJavaScript,
+        this.disableTypeInferenceFlag =
           disableTypeInferenceFlag || !emitJavaScript,
         this.analyzeOnly =
             analyzeOnly || analyzeSignaturesOnly || analyzeAllFlag,