Add Selection.toString() calling the native method

tools/dom/templates/html/impl/impl_Selection.darttemplate

-// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// Copyright (c) 2017, 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.
 
+// WARNING: Do not edit - generated code.
+
 part of $LIBRARYNAME;
+
 $(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS {
-$if DARTIUM

[Alan Knight, 2017/02/03 18:06:37]

Note: this is bogus. The file is a new addition, and something is being
excessively clever and considering it a diff vs. the ImageData template.

-  List<int> __data;
-
-  List<int> get data {
-    if (__data == null) {
-      __data = _data;
-    }
-    return __data;
-  }
+$!MEMBERS
+$if DART2JS
+  @DomName('Selection.toString')
+  @DocsEditable()
+  String toString() native;
 $endif
-
-$!MEMBERS
 }