Model 'await for' as an explicit node in tree and fix its handling.

pkg/compiler/lib/src/use_unused_api.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.
 
 /// This file use methods that aren't used by dart2js.dart, but that we wish to
 /// keep anyway. This might be general API that isn't currently in use,
 /// debugging aids, or API only used for testing (see TODO below).
 
 library dart2js.use_unused_api;
 
@@ skipping 72 matching lines @@
 
 void useConstant(constants.ConstantValue constant,
                  dart2jslib.ConstantSystem cs) {
   constant.isObject;
   cs.isBool(constant);
 }
 
 void useNode(tree.Node node) {
   node
     ..asAsyncModifier()
+    ..asAsyncForIn()
     ..asAwait()
     ..asBreakStatement()
     ..asCascade()
     ..asCatchBlock()
     ..asClassNode()
     ..asCombinator()
     ..asConditional()
     ..asContinueStatement()
     ..asEnum()
     ..asErrorExpression()
@@ skipping 16 matching lines @@
     ..asPartOf()
     ..asRethrow()
     ..asReturn()
     ..asStatement()
     ..asStringInterpolation()
     ..asStringInterpolationPart()
     ..asStringJuxtaposition()
     ..asStringNode()
     ..asSwitchCase()
     ..asSwitchStatement()
+    ..asSyncForIn()
     ..asTryStatement()
     ..asTypeAnnotation()
     ..asTypeVariable()
     ..asTypedef()
     ..asWhile()
     ..asYield();
 }
 
 void useUtil(util.Link link) {
   link.reversePrependAll(link);
@@ skipping 158 matching lines @@
   visitFieldDefinition(tree_ir.FieldDefinition node) {}
 
   visitFieldInitializer(tree_ir.FieldInitializer node) {}
   visitSuperInitializer(tree_ir.SuperInitializer node) {}
 }
 
 useTreeVisitors() {
   new DummyTreeVisitor().visitRootNode(null);
   new DummyTreeVisitor().visitInitializer(null);
 }