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

sdk/lib/_internal/compiler/js_lib/js_string.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.
 
 part of _interceptors;
 
 /**
  * The interceptor class for [String]. The compiler recognizes this
  * class as an interceptor, and changes references to [:this:] to
  * actually use the receiver of the method, which is generated as an extra
@@ skipping 341 matching lines @@
     if (delta <= 0) return this;
     return padding * delta + this;
   }
 
   String padRight(int width, [String padding = ' ']) {
     int delta = width - this.length;
     if (delta <= 0) return this;
     return this + padding * delta;
   }
 
-  List<int> get codeUnits => new _CodeUnits(this);
+  List<int> get codeUnits => new CodeUnits(this);
 
   Runes get runes => new Runes(this);
 
   int indexOf(Pattern pattern, [int start = 0]) {
     checkNull(pattern);
     if (start is! int) throw new ArgumentError(start);
     if (start < 0 || start > this.length) {
       throw new RangeError.range(start, 0, this.length);
     }
     if (pattern is String) {
@@ skipping 76 matching lines @@
   Type get runtimeType => String;
 
   int get length => JS('int', r'#.length', this);
 
   String operator [](int index) {
     if (index is !int) throw new ArgumentError(index);
     if (index >= length || index < 0) throw new RangeError.value(index);
     return JS('String', '#[#]', this, index);
   }
 }
-
-/**
- * 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. */
-  String _string;
-
-  _CodeUnits(this._string);
-
-  int get length => _string.length;
-  int operator[](int i) => _string.codeUnitAt(i);
-}