Reify type params on map literals

test/codegen_expected/language/mixin_prefix_test.js

 dart_library.library('language/mixin_prefix_test', null, /* Imports */[
   'dart_sdk',
   'expect'
 ], function load__mixin_prefix_test(exports, dart_sdk, expect) {
   'use strict';
   const core = dart_sdk.core;
   const convert = dart_sdk.convert;
   const dart = dart_sdk.dart;
   const dartx = dart_sdk.dartx;
   const expect$ = expect.expect;
   const mixin_prefix_test = Object.create(null);
   const mixin_prefix_lib = Object.create(null);
   let VoidTovoid = () => (VoidTovoid = dart.constFn(dart.definiteFunctionType(dart.void, [])))();
   mixin_prefix_lib.MixinClass = class MixinClass extends core.Object {
     bar() {
-      return convert.JSON.encode(dart.map({a: 1}));
+      return convert.JSON.encode(dart.map({a: 1}, core.String, core.int));
     }
   };
   dart.setSignature(mixin_prefix_lib.MixinClass, {
     methods: () => ({bar: dart.definiteFunctionType(core.String, [])})
   });
   mixin_prefix_test.A = class A extends dart.mixin(core.Object, mixin_prefix_lib.MixinClass) {
     baz() {
       return this.bar();
     }
   };
   dart.setSignature(mixin_prefix_test.A, {
     methods: () => ({baz: dart.definiteFunctionType(core.String, [])})
   });
   mixin_prefix_test.main = function() {
     let a = new mixin_prefix_test.A();
     expect$.Expect.equals('{"a":1}', a.baz());
   };
   dart.fn(mixin_prefix_test.main, VoidTovoid());
   // Exports:
   exports.mixin_prefix_test = mixin_prefix_test;
   exports.mixin_prefix_lib = mixin_prefix_lib;
 });