Use failedAt in more places

pkg/compiler/lib/src/tree/nodes.dart

Index: pkg/compiler/lib/src/tree/nodes.dart
diff --git a/pkg/compiler/lib/src/tree/nodes.dart b/pkg/compiler/lib/src/tree/nodes.dart
index 14888c1a4afe555689cbbf04f19a6ebd22a046bb..81416fdb5874acce4ce3c2df9e07a4cfecf418d8 100644
--- a/pkg/compiler/lib/src/tree/nodes.dart
+++ b/pkg/compiler/lib/src/tree/nodes.dart
@@ -3261,8 +3261,8 @@ abstract class NullTreeElementMixin implements TreeElementMixin, Spannable {
   // You're not really supposed to access this field anyways.
   Object get _element => null;
   set _element(_) {
-    assert(invariant(this, false,
-        message: "Elements cannot be associated with ${runtimeType}."));
+    assert(false,
+        failedAt(this, "Elements cannot be associated with ${runtimeType}."));
   }
 }