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

sdk/lib/_internal/compiler/implementation/js_backend/backend.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/js_backend/backend.dart	(revision 15541)
+++ sdk/lib/_internal/compiler/implementation/js_backend/backend.dart	(working copy)
@@ -835,6 +835,12 @@
       result = jsStringClass;
     } else if (cls == compiler.listClass) {
       result = jsArrayClass;
+      // The backend will try to optimize array access and use the
+      // [:ioore:] and [:iae:] helpers directly.
+      enqueuer.registerStaticUse(
+          compiler.findHelper(const SourceString('ioore')));
+      enqueuer.registerStaticUse(
+          compiler.findHelper(const SourceString('iae')));
     } else if (cls == compiler.intClass) {
       result = jsIntClass;
       enqueuer.registerInstantiatedClass(jsNumberClass);