Use failedAt in more places

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:collection' show IterableMixin;
 
 import 'package:front_end/src/fasta/fasta_codes.dart' show FastaMessage;
 import 'package:front_end/src/fasta/scanner.dart' show BeginGroupToken, Token;
 import 'package:front_end/src/fasta/scanner/token_constants.dart' as Tokens
     show PLUS_TOKEN;
@@ skipping 3243 matching lines @@
 
 /// Null implementation of [TreeElementMixin] which does not allow association
 /// of elements.
 ///
 /// This class is the superclass of all AST nodes.
 abstract class NullTreeElementMixin implements TreeElementMixin, Spannable {
   // Deliberately using [Object] here to thwart code completion.
   // 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}."));
   }
 }
 
 /// Actual implementation of [TreeElementMixin] which stores the associated
 /// element in the private field [_element].
 ///
 /// This class is mixed into the node classes that are actually associated with
 /// elements.
 abstract class StoredTreeElementMixin implements TreeElementMixin {
   Object _element;
 }
 
 /**
  * Do not call this method directly.  Instead, use an instance of
  * TreeElements.
  *
  * Using [Object] as return type to thwart code completion.
  */
 Object getTreeElement(TreeElementMixin node) => node._element;
 
 /**
  * Do not call this method directly.  Instead, use an instance of
  * TreeElements.
  */
 void setTreeElement(TreeElementMixin node, Object value) {
   node._element = value;
 }