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

runtime/lib/string_patch.dart

Index: runtime/lib/string_patch.dart
diff --git a/runtime/lib/string_patch.dart b/runtime/lib/string_patch.dart
index 76950644b18fcd51c9e1b457cd1c1f1a0b68eb97..9d938c5b8d13e2bea417c68297960fac72e78eb3 100644
--- a/runtime/lib/string_patch.dart
+++ b/runtime/lib/string_patch.dart
@@ -884,7 +884,7 @@ class _StringBase {
     return result;
   }
 
-  List<int> get codeUnits => new _CodeUnits(this);
+  List<int> get codeUnits => new CodeUnits(this);
 
   Runes get runes => new Runes(this);
 
@@ -1271,17 +1271,3 @@ class _StringMatch implements Match {
   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);
-}