Fix Unicode issues in dart2js and dart2dart.

sdk/lib/_internal/compiler/implementation/util/characters.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.
 
 library characters;
 
 const int $EOF = 0;
 const int $STX = 2;
 const int $BS  = 8;
 const int $TAB = 9;
@@ skipping 115 matching lines @@
   assert(isHexDigit(hexDigit));
   // hexDigit is one of '0'..'9', 'A'..'F' and 'a'..'f'.
   if (hexDigit <= $9) return hexDigit - $0;
   return (hexDigit | ($a ^ $A)) - ($a - 10);
 }
 
 bool isUnicodeScalarValue(int value) {
   return value < $FIRST_SURROGATE ||
       (value > $LAST_SURROGATE && value <= $LAST_CODE_POINT);
 }
+
+bool isUtf16LeadSurrogate(int value) {
+  return value >= 0xd800 && value <= 0xdbff;
+}
+
+bool isUtf16TrailSurrogate(int value) {
+  return value >= 0xdc00 && value <= 0xdfff;
+}