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

sdk/lib/internal/internal.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 dart._internal;
 
 import 'dart:collection';
 
 import 'dart:core' hide Symbol;
 import 'dart:core' as core;
@@ skipping 27 matching lines @@
           100000000000000.0,
          1000000000000000.0,  /*  15 */
         10000000000000000.0,
        100000000000000000.0,
       1000000000000000000.0,
      10000000000000000000.0,
     100000000000000000000.0,  /*  20 */
    1000000000000000000000.0,
   10000000000000000000000.0,
 ];
+
+/**
+ * An [Iterable] of the UTF-16 code units of a [String] in index order.
+ */
+class CodeUnits extends UnmodifiableListBase<int> {
+  /** The string that this is the code units of. */
+  final String _string;
+
+  CodeUnits(this._string);
+
+  int get length => _string.length;
+  int operator[](int i) => _string.codeUnitAt(i);
+
+  static String stringOf(CodeUnits u) => u._string;
+}