Add type inference logic for map literals

pkg/front_end/testcases/inference/map_literals_can_infer_null.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;
 
 test1() {
   var /*@type=Map<Null, Null>*/ x = /*@typeArgs=Null, Null*/ {null: null};
   x /*@target=Map::[]=*/ [
       /*error:INVALID_CAST_LITERAL*/ 3] = /*error:INVALID_CAST_LITERAL*/ 'z';
 }
+
+main() {}