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

sdk/lib/_internal/compiler/js_lib/js_string.dart

Index: sdk/lib/_internal/compiler/js_lib/js_string.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/js_string.dart b/sdk/lib/_internal/compiler/js_lib/js_string.dart
index c7fd9aba51aa0702c034753a901475d7c965e070..67f37d500cedbd5a5f89ac27f7a2a0321f3431fe 100644
--- a/sdk/lib/_internal/compiler/js_lib/js_string.dart
+++ b/sdk/lib/_internal/compiler/js_lib/js_string.dart
@@ -359,7 +359,7 @@ class JSString extends Interceptor implements String, JSIndexable {
     return this + padding * delta;
   }
 
-  List<int> get codeUnits => new _CodeUnits(this);
+  List<int> get codeUnits => new CodeUnits(this);
 
   Runes get runes => new Runes(this);
 
@@ -456,16 +456,3 @@ class JSString extends Interceptor implements String, JSIndexable {
     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);
-}