js_runtime: Split _asyncHelper into separate functions

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 f4bc8518a3267de024a49363974bec2861ea4ca8..4f96c0706687b5c89d01e641f8254057079ebada 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -1291,8 +1291,14 @@ class JavaScriptBackend {
     switch (element.asyncMarker) {
       case AsyncMarker.ASYNC:
         rewriter = new AsyncRewriter(reporter, element,
-            asyncHelper:
-                emitter.staticFunctionAccess(commonElements.asyncHelper),
+            asyncStart:
+                emitter.staticFunctionAccess(commonElements.asyncHelperStart),
+            asyncAwait:
+                emitter.staticFunctionAccess(commonElements.asyncHelperAwait),
+            asyncReturn:
+                emitter.staticFunctionAccess(commonElements.asyncHelperReturn),
+            asyncRethrow:
+                emitter.staticFunctionAccess(commonElements.asyncHelperRethrow),
             wrapBody: emitter.staticFunctionAccess(commonElements.wrapBody),
             completerFactory: emitter
                 .staticFunctionAccess(commonElements.syncCompleterConstructor),