Implementation of async-await transformation on js ast.

pkg/compiler/lib/src/js_backend/backend.dart

Index: pkg/compiler/lib/src/js_backend/backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend.dart b/pkg/compiler/lib/src/js_backend/backend.dart
index 50f645a3f246c9661e4161b2866fec1d2628b26b..cf5a811f847902319eec420cc80a719338a752c7 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -941,6 +941,7 @@ class JavaScriptBackend extends Backend {
       Element e = findHelper('boolConversionCheck');
       if (e != null) enqueue(world, e, registry);
     }
+
     if (TRACE_CALLS) {
       traceHelper = findHelper('traceHelper');
       assert(traceHelper != null);
@@ -1597,6 +1598,60 @@ class JavaScriptBackend extends Backend {
     return findHelper("throwCyclicInit");
   }
 
+  Element getThenHelper() {
+    return findHelper("thenHelper");
+  }
+
+  Element getYieldStar() {
+    ClassElement classElement = findHelper("IterationMarker");
+    classElement.ensureResolved(compiler);
+    return classElement.lookupLocalMember("yieldStar");
+  }
+
+  Element getYieldSingle() {
+    ClassElement classElement = findHelper("IterationMarker");
+    classElement.ensureResolved(compiler);
+    return classElement.lookupLocalMember("yieldSingle");
+  }
+
+  Element getStreamHelper() {
+    return findHelper("streamHelper");
+  }
+
+  Element getEndOfIteration() {
+    ClassElement classElement = findHelper("IterationMarker");
+    classElement.ensureResolved(compiler);
+    return classElement.lookupLocalMember("endOfIteration");
+  }
+
+  Element getSyncStarIterable() {
+    ClassElement classElement = findHelper("SyncStarIterable");
+    classElement.ensureResolved(compiler);
+    return classElement;
+  }
+
+  Element getSyncStarIterableConstructor() {
+    ClassElement classElement = getSyncStarIterable();
+    classElement.ensureResolved(compiler);
+    return classElement.lookupConstructor("");
+  }
+
+  Element getCompleterConstructor() {
+    ClassElement classElement = find(compiler.asyncLibrary, "Completer");
+    classElement.ensureResolved(compiler);
+    return classElement.lookupConstructor("");
+  }
+
+  Element getMakeController() {
+    return findHelper("makeAsyncStarController");
+  }
+
+  Element getStreamIteratorConstructor() {
+    ClassElement classElement = find(compiler.asyncLibrary, "StreamIterator");
+    classElement.ensureResolved(compiler);
+    return classElement.lookupConstructor("");
+  }
+
   bool isNullImplementation(ClassElement cls) {
     return cls == jsNullClass;
   }
@@ -2331,6 +2386,27 @@ class JavaScriptBackend extends Backend {
     String extension = addExtension ? ".part.js" : "";
     return "${outName}_$name$extension";
   }
+
+  void registerAsyncMarker(FunctionElement element,
+                           Enqueuer enqueuer,
+                           Registry registry) {
+    if (element.asyncMarker == AsyncMarker.ASYNC) {
+      enqueue(enqueuer, getThenHelper(), registry);
+      enqueue(enqueuer, getCompleterConstructor(), registry);
+      enqueue(enqueuer, getStreamIteratorConstructor(), registry);
+    } else if (element.asyncMarker == AsyncMarker.SYNC_STAR) {
+      enqueuer.registerInstantiatedClass(getSyncStarIterable(), registry);
+      enqueue(enqueuer, getSyncStarIterableConstructor(), registry);
+      enqueue(enqueuer, getEndOfIteration(), registry);
+      enqueue(enqueuer, getYieldStar(), registry);
+    } else if (element.asyncMarker == AsyncMarker.ASYNC_STAR) {
+      enqueue(enqueuer, getStreamHelper(), registry);
+      enqueue(enqueuer, getYieldSingle(), registry);
+      enqueue(enqueuer, getYieldStar(), registry);
+      enqueue(enqueuer, getMakeController(), registry);
+      enqueue(enqueuer, getStreamIteratorConstructor(), registry);
+    }
+  }
 }
 
 class JavaScriptResolutionCallbacks extends ResolutionCallbacks {