Allow surrogates in string literals.

tests/language/string_unicode4_negative_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.
 
 class StringUnicode4NegativeTest {
 
   static testMain() {
-    // Unicode escapes must refer to valid Unicode points and not surrogate characters
+    // Unicode escapes must refer to valid Unicode points.
     String str = "Foo\u{FFFFFF}";
-    str = "Foo\uD800";
-    str = "Foo\uDC00";
   }
 }
 
 main() {
   StringUnicode4NegativeTest.testMain();
 }