Fix another string concatenation.

lib/dom/templates/html/impl/impl_Element.darttemplate

Index: lib/dom/templates/html/impl/impl_Element.darttemplate
diff --git a/lib/dom/templates/html/impl/impl_Element.darttemplate b/lib/dom/templates/html/impl/impl_Element.darttemplate
index 59e2dfa2ef013d261032a58bb4b7ff125b898c45..924fef7edb99e6cafbe03852f6dd855a896bddbe 100644
--- a/lib/dom/templates/html/impl/impl_Element.darttemplate
+++ b/lib/dom/templates/html/impl/impl_Element.darttemplate
@@ -767,7 +767,7 @@ class _ElementFactoryProvider {
       // 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();