Properly type infer for-in loops when the iterator type is a type parameter.

pkg/front_end/testcases/inference/infer_types_on_loop_indices_for_each_loop.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;
 
 class Foo {
   int bar = 42;
 }
@@ skipping 49 matching lines @@
   for (var /*@type=dynamic*/ x in /*error:FOR_IN_OF_INVALID_TYPE*/ map) {
     String y = /*info:DYNAMIC_CAST*/ x;
   }
 
   // We're not properly inferring that map.keys is an Iterable<String>
   // and that x is a String.
   for (var /*@type=String*/ x in map. /*@target=Map::keys*/ keys) {
     String y = x;
   }
 }
+
+main() {}