Add Selection.toString() calling the native method

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

Index: tools/dom/templates/html/impl/impl_Selection.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_ImageData.darttemplate b/tools/dom/templates/html/impl/impl_Selection.darttemplate
similarity index 58%
copy from tools/dom/templates/html/impl/impl_ImageData.darttemplate
copy to tools/dom/templates/html/impl/impl_Selection.darttemplate
index dd04deedcc784a23b0ac47e56b21526f604ee431..ed9f52abefdd5a55187958812e98715aaa2dbf38 100644
--- a/tools/dom/templates/html/impl/impl_ImageData.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Selection.darttemplate
@@ -1,19 +1,16 @@
-// 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.
 
-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;
+// WARNING: Do not edit - generated code.
 
-  List<int> get data {
-    if (__data == null) {
-      __data = _data;
-    }
-    return __data;
-  }
-$endif
+part of $LIBRARYNAME;
 
+$(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS {
 $!MEMBERS
+$if DART2JS
+  @DomName('Selection.toString')
+  @DocsEditable()
+  String toString() native;
+$endif
 }