Add some support for the code-point code-unit distinction.

sdk/lib/utf/utf8.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.
 
 const int _UTF8_ONE_BYTE_MAX = 0x7f;
 const int _UTF8_TWO_BYTE_MAX = 0x7ff;
 const int _UTF8_THREE_BYTE_MAX = 0xffff;
 
 const int _UTF8_LO_SIX_BIT_MASK = 0x3f;
 
@@ skipping 23 matching lines @@
 
 /**
  * Produce a String from a List of UTF-8 encoded bytes. The parameters
  * can set an offset into a list of bytes (as int), limit the length of the
  * values to be decoded, and override the default Unicode replacement character.
  * Set the replacementCharacter to null to throw an ArgumentError
  * rather than replace the bad value.
  */
 String decodeUtf8(List<int> bytes, [int offset = 0, int length,
     int replacementCodepoint = UNICODE_REPLACEMENT_CHARACTER_CODEPOINT]) {
-  return codepointsToString(
+  return new String.fromCharCodes(
       (new Utf8Decoder(bytes, offset, length, replacementCodepoint))
       .decodeRest());
 }
 
 /**
  * Produce a sequence of UTF-8 encoded bytes from the provided string.
  */
 List<int> encodeUtf8(String str) =>
-  codepointsToUtf8(stringToCodepoints(str));
+  codepointsToUtf8(str.charCodes);
 
 int _addToEncoding(int offset, int bytes, int value, List<int> buffer) {
   while (bytes > 0) {
     buffer[offset + bytes] = _UTF8_SUBSEQUENT_BYTE_BASE |
         (value & _UTF8_LO_SIX_BIT_MASK);
     value = value >> 6;
     bytes--;
   }
   return value;
 }
@@ skipping 187 matching lines @@
     if (validSequence && nonOverlong && inRange) {
       return value;
     } else if (replacementCodepoint != null) {
       return replacementCodepoint;
     } else {
       throw new ArgumentError(
           "Invalid UTF8 at ${utf8EncodedBytesIterator.position - j}");
     }
   }
 }