Add a new magic element type for annotating types that have Javascript array behavior.

lib/compiler/implementation/lib/js_helper.dart

Index: lib/compiler/implementation/lib/js_helper.dart
===================================================================
--- lib/compiler/implementation/lib/js_helper.dart	(revision 8591)
+++ lib/compiler/implementation/lib/js_helper.dart	(working copy)
@@ -1088,3 +1088,11 @@
   if (JS('bool', '#[#]()', value, property)) return value;
   propertyTypeError(value, property);
 }
+
+/**
+ * Special interface recognized by the compiler and implemented by DOM
+ * objects that support integer indexing. This interface is not
+ * visible to anyone, and is only injected into special libraries.
+ */
+interface JavaScriptIndexingBehavior {
+}