dart2js cps: Support async/await by rewriting the JavaScript AST.

pkg/compiler/lib/src/js_backend/codegen/task.dart

 // Copyright (c) 2014, 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.
 
 /// Generate code using the cps-based IR pipeline.
 library code_generator_task;
 
 import 'glue.dart';
 import 'codegen.dart';
 import 'unsugar.dart';
@@ skipping 16 matching lines @@
 import '../../tracer.dart';
 import '../../js_backend/codegen/codegen.dart';
 import '../../ssa/ssa.dart' as ssa;
 import '../../tree_ir/optimization/optimization.dart';
 import '../../tree_ir/optimization/optimization.dart' as tree_opt;
 import '../../tree_ir/tree_ir_integrity.dart';
 import '../../cps_ir/cps_ir_nodes_sexpr.dart';
 
 class CpsFunctionCompiler implements FunctionCompiler {
   final ConstantSystem constantSystem;
+  // TODO(karlklose): remove the compiler.
   final Compiler compiler;
   final Glue glue;
   final SourceInformationStrategy sourceInformationFactory;
 
   // TODO(karlklose,sigurm): remove and update dart-doc of [compile].
   final FunctionCompiler fallbackCompiler;
 
   Tracer get tracer => compiler.tracer;
 
   IrBuilderTask get irBuilderTask => compiler.irBuilder;
 
   CpsFunctionCompiler(Compiler compiler, JavaScriptBackend backend,
                       SourceInformationStrategy sourceInformationFactory)
       : fallbackCompiler =
             new ssa.SsaFunctionCompiler(backend, sourceInformationFactory),
         this.sourceInformationFactory = sourceInformationFactory,
         constantSystem = backend.constantSystem,
         compiler = compiler,
         glue = new Glue(compiler);
 
   String get name => 'CPS Ir pipeline';
 
+  JavaScriptBackend get backend => compiler.backend;
+
   /// Generates JavaScript code for `work.element`.
   js.Fun compile(CodegenWorkItem work) {
     AstElement element = work.element;
     JavaScriptBackend backend = compiler.backend;
     return compiler.withCurrentElement(element, () {
       try {
         // TODO(karlklose): remove this fallback.
         // Fallback for a few functions that we know require try-finally and
         // switch.
         if (element.isNative) {
@@ skipping 137 matching lines @@
     applyTreePass(new LoopRewriter());
     applyTreePass(new LogicalRewriter());
     applyTreePass(new PullIntoInitializers());
 
     return node;
   }
 
   js.Fun compileToJavaScript(CodegenWorkItem work,
                              tree_ir.FunctionDefinition definition) {
     CodeGenerator codeGen = new CodeGenerator(glue, work.registry);
-    return attachPosition(codeGen.buildFunction(definition), work.element);
+    Element element = work.element;
+    js.Fun code = codeGen.buildFunction(definition);
+    if (element is FunctionElement && element.asyncMarker != AsyncMarker.SYNC) {
+      code = backend.rewriteAsync(element, code);
+      work.registry.registerAsyncMarker(element);
+    }
+    return attachPosition(code, element);
   }
 
   Iterable<CompilerTask> get tasks {
     // TODO(sigurdm): Make a better list of tasks.
     return <CompilerTask>[irBuilderTask]..addAll(fallbackCompiler.tasks);
   }
 
   js.Node attachPosition(js.Node node, AstElement element) {
     return node.withSourceInformation(
         sourceInformationFactory.createBuilderForContext(element)
             .buildDeclaration(element));
   }
 }