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

lib/compiler/implementation/compiler.dart

Index: lib/compiler/implementation/compiler.dart
===================================================================
--- lib/compiler/implementation/compiler.dart	(revision 8103)
+++ lib/compiler/implementation/compiler.dart	(working copy)
@@ -108,6 +108,13 @@
   ClassElement nullClass;
   ClassElement listClass;
 
+  /**
+   * Interface used to determine if an object has the JavaScript
+   * indexing behavior. The interface is only visible to specific
+   * libraries.
+   */
+  ClassElement javaScriptIndexingBehaviorInterface;

[ahe, 2012/06/12 18:30:04]

How about jsIndexingBehaviorInterface?

[ngeoffray, 2012/06/13 13:02:42]

Done.

+
   Element get currentElement() => _currentElement;
   withCurrentElement(Element element, f()) {
     Element old = currentElement;
@@ -329,6 +336,9 @@
     libraries['dart:coreimpl'] = coreImplLibrary;
 
     initializeSpecialClasses();
+
+    javaScriptIndexingBehaviorInterface =
+        findHelper(const SourceString('JavaScriptIndexingBehavior'));
   }
 
   /** Define the JS helper functions in the given library. */