dart2js: Refactoring, documentation, and a few bugfixes in Namer class.

pkg/compiler/lib/src/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 123 matching lines @@
       (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;
 }
+
+bool isDigit(int characterCode) {
+  return $0 <= characterCode && characterCode <= $9;
+}