Require two type arguments for map literals (issue 4522).

tests/language/map_literal4_test.dart

 // Copyright (c) 2011, 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.
 // VMOptions=--enable_type_checks
 //
 // Dart test program testing type checks in map literals.
 
 class MapLiteral4Test<T> {
   test() {
     int result = 0;
     try {
-      var m = <String>{"a": 0};  // 0 is not a String.
+      var m = <String, String>{"a": 0};  // 0 is not a String.
     } catch (TypeError error) {
       result += 1;
     }
     try {
-      var m = <int>{"a": 0};
+      var m = <String, int>{"a": 0};
       m[2] = 1;  // 2 is not a String.
     } catch (TypeError error) {
       result += 10;
     }
     try {
-      var m = <T>{"a": "b"};  // "b" is not an int.
+      var m = <String, T>{"a": "b"};  // "b" is not an int.
     } catch (TypeError error) {
       result += 100;
     }
     try {
-      var m = <T>{"a": 0};  // OK.
+      var m = <String, T>{"a": 0};  // OK.
     } catch (TypeError error) {
       result += 1000;
     }
     try {
-      var m = <T>{"a": 0};
+      var m = <String, T>{"a": 0};
       m[2] = 1;  // 2 is not a String.
     } catch (TypeError error) {
       result += 10000;
     }
     try {
-      var m = const <int>{"a": 0};
+      var m = const <String, int>{"a": 0};
       m[2] = 1;  // 2 is not a String.
     } catch (TypeError error) {
       result += 100000;
     }
     return result;
   }
 }
 
 main() {
   var t = new MapLiteral4Test<int>();
   Expect.equals(110111, t.test());
 }