Add optional arguments to our indexOf/lastIndexOf methods.

client/html/generated/src/wrapping/_MediaListWrappingImplementation.dart

Index: client/html/generated/src/wrapping/_MediaListWrappingImplementation.dart
===================================================================
--- client/html/generated/src/wrapping/_MediaListWrappingImplementation.dart	(revision 942)
+++ client/html/generated/src/wrapping/_MediaListWrappingImplementation.dart	(working copy)
@@ -41,12 +41,13 @@
     throw new UnsupportedOperationException("This object is immutable.");
   }
 
-  int indexOf(String element, int startIndex) {
-    return _Lists.indexOf(this, element, startIndex, this.length);
+  int indexOf(String element, [int start = 0]) {
+    return _Lists.indexOf(this, element, start, this.length);
   }
 
-  int lastIndexOf(String element, int startIndex) {
-    return _Lists.lastIndexOf(this, element, startIndex);
+  int lastIndexOf(String element, [int start = null]) {
+    if (start === null) start = length - 1;
+    return _Lists.lastIndexOf(this, element, start);
   }
 
   int clear() {