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 16030)
+++ sdk/lib/_internal/compiler/implementation/js_backend/backend.dart	(working copy)
@@ -849,6 +849,12 @@
       addInterceptors(jsStringClass, enqueuer);
     } else if (cls == compiler.listClass) {
       addInterceptors(jsArrayClass, enqueuer);
+      // 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) {
       addInterceptors(jsIntClass, enqueuer);
       addInterceptors(jsNumberClass, enqueuer);