Restrict backend misuse

pkg/compiler/lib/src/js_backend/backend_helpers.dart

Index: pkg/compiler/lib/src/js_backend/backend_helpers.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend_helpers.dart b/pkg/compiler/lib/src/js_backend/backend_helpers.dart
index d0b2750262d5302414cd9fa42858710d6f90a256..f0a92d7581aac099ee5a40faaa7effe1302a0a2e 100644
--- a/pkg/compiler/lib/src/js_backend/backend_helpers.dart
+++ b/pkg/compiler/lib/src/js_backend/backend_helpers.dart
@@ -6,6 +6,7 @@ library dart2js.js_backend.helpers;
 
 import '../common.dart';
 import '../common/names.dart' show
+    Identifiers,
     Uris;
 import '../common/resolution.dart' show
     Resolution;
@@ -679,4 +680,18 @@ class BackendHelpers {
   Element get convertRtiToRuntimeType {
     return findHelper('convertRtiToRuntimeType');
   }
+
+  ClassElement get stackTraceClass {
+    return findHelper('_StackTrace');
+  }
+
+  MethodElement _objectNoSuchMethod;
+
+  MethodElement get objectNoSuchMethod {
+    if (_objectNoSuchMethod == null) {
+      _objectNoSuchMethod =
+          coreClasses.objectClass.lookupLocalMember(Identifiers.noSuchMethod_);
+    }
+    return _objectNoSuchMethod;
+  }
 }