Let Uint16List.setRange detect a String.codeUnits object, and copy directly from the string.

runtime/lib/string_patch.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 _maxAscii = 0x7f;
 const int _maxLatin1 = 0xff;
 const int _maxUtf16 = 0xffff;
 const int _maxUnicode = 0x10ffff;
 
 patch class String {
@@ skipping 866 matching lines @@
       if (startIndex == endIndex && endIndex == previousIndex) {
         ++startIndex;  // empty match, advance and restart
         continue;
       }
       result.add(this._substringUnchecked(previousIndex, match.start));
       startIndex = previousIndex = endIndex;
     }
     return result;
   }
 
-  List<int> get codeUnits => new _CodeUnits(this);
+  List<int> get codeUnits => new CodeUnits(this);
 
   Runes get runes => new Runes(this);
 
   String toUpperCase() native "String_toUpperCase";
 
   String toLowerCase() native "String_toLowerCase";
 
   // Concatenate ['start', 'end'[ elements of 'strings'. 'strings' must contain
   // String elements. TODO(srdjan): optimize it.
   static String _concatRange(List<String> strings, int start, int end) {
@@ skipping 366 matching lines @@
     for (int g in groups) {
       result.add(group(g));
     }
     return result;
   }
 
   final int start;
   final String input;
   final String pattern;
 }
-
-/**
- * An [Iterable] of the UTF-16 code units of a [String] in index order.
- */
-class _CodeUnits extends Object with ListMixin<int>,
-                                     UnmodifiableListMixin<int> {
-  /** The string that this is the code units of. */
-  String _string;
-
-  _CodeUnits(this._string);
-
-  int get length => _string.length;
-  int operator[](int i) => _string.codeUnitAt(i);
-}