Add type inference logic for map literals

pkg/front_end/testcases/inference/map_literals_top_level.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;
 
 var /*@topType=Map<int, String>*/ x1 = /*@typeArgs=int, String*/ {
   1: 'x',
   2: 'y'
@@ skipping 13 matching lines @@
 };
 test2() {
   x2 /*@target=Map::[]=*/ [3] = 'z';
   x2 /*@target=Map::[]=*/ [/*error:ARGUMENT_TYPE_NOT_ASSIGNABLE*/ 'hi'] = 'w';
   x2 /*@target=Map::[]=*/ [4.0] = 'u';
   x2 /*@target=Map::[]=*/ [3] = /*error:INVALID_ASSIGNMENT*/ 42;
   Pattern p = null;
   x2 /*@target=Map::[]=*/ [2] = p;
   Map<int, String> y = /*info:ASSIGNMENT_CAST*/ x2;
 }
+
+main() {}