Move the handling of operator[] into the new interceptors.

sdk/lib/_internal/compiler/implementation/lib/js_array.dart

Index: sdk/lib/_internal/compiler/implementation/lib/js_array.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/lib/js_array.dart	(revision 16030)
+++ sdk/lib/_internal/compiler/implementation/lib/js_array.dart	(working copy)
@@ -181,4 +181,10 @@
     checkGrowable(this, 'set length');
     JS('void', r'#.length = #', this, newLength);
   }
+
+  E operator [](int index) {
+    if (index is !int) throw new ArgumentError(index);
+    if (index >= length || index < 0) throw new RangeError.value(index);
+    return JS('var', '#[#]', this, index);
+  }
 }