Tweak toString() for better type inference

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

 // 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 $LIBRARYNAME;
 
 /**
  * Lazy implementation of the child nodes of an element that does not request
  * the actual child nodes of an element until strictly necessary greatly
  * improving performance for the typical cases where it is not required.
@@ skipping 254 matching lines @@
 
   void _clearChildren() {
     while (firstChild != null) {
       _removeChild(firstChild);
     }
   }
 
   /**
    * Print out a String representation of this Node.
    */
-  String toString() => nodeValue == null ? super.toString() : nodeValue;
+  String toString() {
+    String value = nodeValue;  // Fetch DOM Node property once.
+    return value == null ? super.toString() : value;
+  }
 $!MEMBERS
 }