Reify type params on map literals

test/codegen_expected/corelib/expando_test.js

 dart_library.library('corelib/expando_test', null, /* Imports */[
   'dart_sdk',
   'expect'
 ], function load__expando_test(exports, dart_sdk, expect) {
   'use strict';
   const core = dart_sdk.core;
   const _interceptors = dart_sdk._interceptors;
   const dart = dart_sdk.dart;
   const dartx = dart_sdk.dartx;
   const expect$ = expect.expect;
   const expando_test = Object.create(null);
   let ExpandoOfint = () => (ExpandoOfint = dart.constFn(core.Expando$(core.int)))();
   let JSArrayOfint = () => (JSArrayOfint = dart.constFn(_interceptors.JSArray$(core.int)))();
   let JSArrayOfObject = () => (JSArrayOfObject = dart.constFn(_interceptors.JSArray$(core.Object)))();
   let VoidToint = () => (VoidToint = dart.constFn(dart.definiteFunctionType(core.int, [])))();
   let dynamicTobool = () => (dynamicTobool = dart.constFn(dart.definiteFunctionType(core.bool, [dart.dynamic])))();
   let VoidTodynamic = () => (VoidTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [])))();
   let const$;
   let const$0;
   expando_test.ExpandoTest = class ExpandoTest extends core.Object {
     static testMain() {
       expando_test.ExpandoTest.visits = new (ExpandoOfint())('visits');
-      let legal = JSArrayOfObject().of([new core.Object(), core.List.new(), JSArrayOfint().of([1, 2, 3]), const$ || (const$ = dart.constList([1, 2, 3], core.int)), core.Map.new(), dart.map({x: 1, y: 2}), const$0 || (const$0 = dart.const(dart.map({x: 1, y: 2}))), new core.Expando(), new core.Expando('horse')]);
+      let legal = JSArrayOfObject().of([new core.Object(), core.List.new(), JSArrayOfint().of([1, 2, 3]), const$ || (const$ = dart.constList([1, 2, 3], core.int)), core.Map.new(), dart.map({x: 1, y: 2}, core.String, core.int), const$0 || (const$0 = dart.const(dart.map({x: 1, y: 2}, core.String, core.int))), new core.Expando(), new core.Expando('horse')]);
       for (let object of legal) {
         expando_test.ExpandoTest.testNamedExpando(object);
         expando_test.ExpandoTest.testUnnamedExpando(object);
       }
       for (let object of legal) {
         expect$.Expect.equals(2, expando_test.ExpandoTest.visits.get(object), dart.str`${object}`);
       }
       expando_test.ExpandoTest.testIllegal();
       expando_test.ExpandoTest.testIdentity();
     }
@@ skipping 83 matching lines @@
     ['=='](other) {
       return expando_test.Mutable.is(other) && other.id == this.id;
     }
   };
   dart.setSignature(expando_test.Mutable, {
     constructors: () => ({new: dart.definiteFunctionType(expando_test.Mutable, [core.int])})
   });
   // Exports:
   exports.expando_test = expando_test;
 });