Add optional arguments to our indexOf/lastIndexOf methods.

client/html/src/DocumentFragmentWrappingImplementation.dart

Index: client/html/src/DocumentFragmentWrappingImplementation.dart
===================================================================
--- client/html/src/DocumentFragmentWrappingImplementation.dart	(revision 942)
+++ client/html/src/DocumentFragmentWrappingImplementation.dart	(working copy)
@@ -99,10 +99,14 @@
   Iterator<Element> iterator() => _filtered.iterator();
   List<Element> getRange(int start, int length) =>
     _filtered.getRange(start, length);
-  int indexOf(Element element, int startIndex) =>
+  int indexOf(Element element, [int startIndex = 0]) =>
     _filtered.indexOf(element, startIndex);
-  int lastIndexOf(Element element, int startIndex) =>
-    _filtered.lastIndexOf(element, startIndex);
+
+  int lastIndexOf(Element element, [int fromIndex = null]) {
+    if (fromIndex === null) fromIndex = length - 1;
+    return _filtered.lastIndexOf(element, fromIndex);
+  }
+
   Element last() => _filtered.last();
 }
 
@@ -362,4 +366,4 @@
     throw new UnsupportedOperationException(
       "WebKit drop zone can't be set for document fragments.");
   }
-}
+}