Support async/await in dart2dart.

pkg/compiler/lib/src/compiler.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart2js;
 
 /**
  * If true, print a warning for each method that was resolved, but not
  * compiled.
  */
@@ skipping 725 matching lines @@
 
   /// Map containing information about the warnings and hints that have been
   /// suppressed for each library.
   Map<Uri, SuppressionInfo> suppressedWarnings = <Uri, SuppressionInfo>{};
 
   final bool suppressWarnings;
 
   /// `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;
 
   /// If `true` native extension syntax is supported by the frontend.
   final bool allowNativeExtensions;
 
   /// Output provider from user of Compiler API.
   api.CompilerOutputProvider userOutputProvider;
 
@@ skipping 241 matching lines @@
             this.outputUri: null,
             this.buildId: UNDETERMINED_BUILD_ID,
             this.terseDiagnostics: false,
             this.deferredMapUri: null,
             this.dumpInfo: false,
             this.showPackageWarnings: false,
             this.useContentSecurityPolicy: false,
             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,
         this.analyzeSignaturesOnly = analyzeSignaturesOnly,
         this.analyzeAllFlag = analyzeAllFlag,
         this.hasIncrementalSupport = hasIncrementalSupport,
         cacheStrategy = new CacheStrategy(hasIncrementalSupport),
         this.userOutputProvider = (outputProvider == null)
             ? NullSink.outputProvider
             : outputProvider {
@@ skipping 1363 matching lines @@
   InterfaceType get nullType => nullClass.computeType(compiler);
 
   @override
   InterfaceType get numType => numClass.computeType(compiler);
 
   @override
   InterfaceType get stringType =>  stringClass.computeType(compiler);
 }
 
 typedef void InternalErrorFunction(Spannable location, String message);