Make some ErrorCode-s compile-time errors and some just type warnings

tests/language/src/WrongNumberTypeArgumentsNegativeTest.dart

Index: tests/language/src/WrongNumberTypeArgumentsNegativeTest.dart
diff --git a/tests/language/src/WrongNumberTypeArgumentsNegativeTest.dart b/tests/language/src/WrongNumberTypeArgumentsNegativeTest.dart
index 090da1dac84886bfc01ef42d90261d4539cbf338..44795e3fea7fb265313df48d1958202ed80bb036 100644
--- a/tests/language/src/WrongNumberTypeArgumentsNegativeTest.dart
+++ b/tests/language/src/WrongNumberTypeArgumentsNegativeTest.dart
@@ -2,8 +2,6 @@
 // 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.
 
-Map<String> foo;  // Error: Map takes 2 type arguments.
-
 main() {
-  foo = null;
+  foo = new Map<String>(); // Error: Map takes 2 type arguments.
 }