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

tests/language/map_literal4_test.dart

Index: tests/language/map_literal4_test.dart
===================================================================
--- tests/language/map_literal4_test.dart	(revision 10854)
+++ tests/language/map_literal4_test.dart	(working copy)
@@ -9,34 +9,34 @@
   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;