Create ResolutionEnqueuer after library loading.

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

Index: pkg/compiler/lib/src/js_backend/backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend.dart b/pkg/compiler/lib/src/js_backend/backend.dart
index be709f53ef62a823ed779c33bc2f6ada50247ca2..9afc4a31eb1942eb6eed83dd59de6e6d8c8f9738 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -831,6 +831,14 @@ class JavaScriptBackend {
     }
   }
 
+  void onResolutionStart(ResolutionEnqueuer enqueuer) {
+    helpers.onResolutionStart();
+
+    validateInterceptorImplementsAllObjectMethods(helpers.jsInterceptorClass);
+    // The null-interceptor must also implement *all* methods.
+    validateInterceptorImplementsAllObjectMethods(helpers.jsNullClass);
+  }
+
   void onResolutionComplete(
       ClosedWorld closedWorld, ClosedWorldRefiner closedWorldRefiner) {
     for (Entity entity in compiler.enqueuer.resolution.processedEntities) {
@@ -1177,10 +1185,6 @@ class JavaScriptBackend {
       ..add(helpers.jsInterceptorClass)
       ..add(helpers.jsNullClass);
 
-    validateInterceptorImplementsAllObjectMethods(helpers.jsInterceptorClass);
-    // The null-interceptor must also implement *all* methods.
-    validateInterceptorImplementsAllObjectMethods(helpers.jsNullClass);
-
     return new Future.value();
   }