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

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

Index: pkg/compiler/lib/src/js_backend/codegen/task.dart
diff --git a/pkg/compiler/lib/src/js_backend/codegen/task.dart b/pkg/compiler/lib/src/js_backend/codegen/task.dart
index 14cc633582ddab1c59fb77b351d27f1dcd54554d..d784197b177673efbf2cfc3424983d78a736760d 100644
--- a/pkg/compiler/lib/src/js_backend/codegen/task.dart
+++ b/pkg/compiler/lib/src/js_backend/codegen/task.dart
@@ -34,6 +34,7 @@ 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;
@@ -56,6 +57,8 @@ class CpsFunctionCompiler implements FunctionCompiler {
 
   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;
@@ -213,7 +216,13 @@ class CpsFunctionCompiler implements FunctionCompiler {
   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 {