restructure string decoding to support iterable use and include benchmarks for UTF-8 decoding.

utils/string_encoding/unicode.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.
 
 #library("unicode");
 #import("unicode_core.dart");
 
 /**
- * Provide Unicode codepoints for a given string.
+ * Provide a list of Unicode codepoints for a given string.
  */
 List<int> stringToCodepoints(String str) {
   List<int> codepoints;
   // TODO is16BitCodeUnit() is used to work around a bug with frog/dartc
   // (http://code.google.com/p/dart/issues/detail?id=1357). Consider
   // removing after this issue is resolved.
   if (is16BitCodeUnit()) {
     codepoints = utf16CodeUnitsToCodepoints(str.charCodes());
   } else {
     codepoints = str.charCodes();
   }
   return codepoints;
 }
 
 /**
- * Generate a string for the provided Unicode codepoints.
+ * Generate a string from the provided Unicode codepoints.
  */
 String codepointsToString(List<int> codepoints) {
   // TODO is16BitCodeUnit() is used to work around a bug with frog/dartc
   // (http://code.google.com/p/dart/issues/detail?id=1357). Consider
   // removing after this issue is resolved.
   if (is16BitCodeUnit()) {
     return new String.fromCharCodes(
         codepointsToUtf16CodeUnits(codepoints));
   } else {
     return new String.fromCharCodes(codepoints);
   }
 }