Fix another string concatenation.

lib/dom/templates/html/impl/impl_Element.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.
 
 // TODO(jacobr): use _Lists.dart to remove some of the duplicated
 // functionality.
 class _ChildrenElementList implements ElementList {
   // Raw Element.
   final _ElementImpl _element;
   final _HTMLCollectionImpl _childElements;
@@ skipping 749 matching lines @@
 
     Element element;
     if (temp.elements.length == 1) {
       element = temp.elements.first;
     } else if (parentTag == 'html' && temp.elements.length == 2) {
       // Work around for edge case in WebKit and possibly other browsers where
       // both body and head elements are created even though the inner html
       // only contains a head or body element.
       element = temp.elements[tag == 'head' ? 0 : 1];
     } else {
-      throw new IllegalArgumentException('HTML had ${temp.elements.length} ' +
+      throw new IllegalArgumentException('HTML had ${temp.elements.length} '
           'top level elements but 1 expected');
     }
     element.remove();
     return element;
   }
 
   /** @domName Document.createElement */
 $if FROG
   // Optimization to improve performance until the frog compiler inlines this
   // method.
   factory Element.tag(String tag) native "return document.createElement(tag)";
 $else
   factory Element.tag(String tag) => _document.$dom_createElement(tag);
 $endif
 }