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

utils/tests/string_encoding/unicode_tests.dart

Index: utils/tests/string_encoding/unicode_tests.dart
diff --git a/utils/tests/string_encoding/unicode_tests.dart b/utils/tests/string_encoding/unicode_tests.dart
index 32c0f9c89921a33b272b6333ee17cf1b72b889ed..53114e4b27c5502eaecfaa1b07c71cf9d481b3b7 100755
--- a/utils/tests/string_encoding/unicode_tests.dart
+++ b/utils/tests/string_encoding/unicode_tests.dart
@@ -32,6 +32,6 @@ class UnicodeTests extends TestClass {
   }
 
   void testCodepointsToString() {
-    Expect.listEquals(testPhrase, codepointsToString(testCodepoints));
+    Expect.stringEquals(testPhrase, codepointsToString(testCodepoints));
   }
 }