Add type inference logic for "for-in" loops.

pkg/front_end/testcases/inference/downwards_inference_for_each.dart

 // Copyright (c) 2017, 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.
 
 /*@testedFeatures=inference*/
 library test;
 
 import 'dart:async';
 
 abstract class MyStream<T> extends Stream<T> {
   factory MyStream() => null;
 }
 
+T F<T>() => null;
+
+Future f() async {
+  dynamic d;
+  Object o;
+  for (var /*@type=dynamic*/ x in /*@typeArgs=dynamic*/ F()) {}
+  for (dynamic x in /*@typeArgs=Iterable<dynamic>*/ F()) {}
+  for (Object x in /*@typeArgs=Iterable<Object>*/ F()) {}
+  for (d in /*@typeArgs=dynamic*/ F()) {}
+  for (o in /*@typeArgs=dynamic*/ F()) {}
+  await for (var /*@type=dynamic*/ x in /*@typeArgs=dynamic*/ F()) {}
+  await for (dynamic x in /*@typeArgs=Stream<dynamic>*/ F()) {}
+  await for (Object x in /*@typeArgs=Stream<Object>*/ F()) {}
+  await for (d in /*@typeArgs=dynamic*/ F()) {}
+  await for (o in /*@typeArgs=dynamic*/ F()) {}
+}
+
 Future main() async {
   for (int x in /*@typeArgs=int*/ [1, 2, 3]) {}
+  for (num x in /*@typeArgs=num*/ [1, 2, 3]) {}
+  for (var /*@type=int*/ x in /*@typeArgs=int*/ [1, 2, 3]) {}
   await for (int x in new /*@typeArgs=int*/ MyStream()) {}
+  await for (var /*@type=int*/ x in new MyStream<int>()) {}
 }