Unify most of our utf8 implementations. This takes the implementation

utils/string_encoding/utf8.dart

Index: utils/string_encoding/utf8.dart
===================================================================
--- utils/string_encoding/utf8.dart	(revision 5167)
+++ utils/string_encoding/utf8.dart	(working copy)
@@ -1,8 +0,0 @@
-// 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("utf8");
-#import("unicode_core.dart");
-#import("unicode.dart");
-#source("utf8_impl.dart");