Optimize special cases in utf8 and utf16 where input can be copied to output.

utils/tests/string_encoding/unicode_tests.dart

 #!/usr/bin/env dart
 // Copyright (c) 2012, 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.
 
 #library("unicode_tests");
 #import("dunit.dart");
 #import("../../string_encoding/unicode.dart");
 
 void main() {
@@ skipping 14 matching lines @@
   void registerTests(TestSuite suite) {
     register("testCodepointsToString", testCodepointsToString, suite);
     register("testStringToCodepoints", testStringToCodepoints, suite);
   }
 
   void testStringToCodepoints() {
     Expect.listEquals(testCodepoints, stringToCodepoints(testPhrase));
   }
 
   void testCodepointsToString() {
-    Expect.listEquals(testPhrase, codepointsToString(testCodepoints));
+    Expect.stringEquals(testPhrase, codepointsToString(testCodepoints));
   }
 }