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 Message;
 import 'package:front_end/src/fasta/scanner.dart' show Token;
 import 'package:front_end/src/fasta/scanner/token_constants.dart' as Tokens
     show PLUS_TOKEN;
@@ skipping 2142 matching lines @@
     return isInterpolationCache;
   }
 
   /**
    * Retrieve a single DartString that represents this entire juxtaposition
    * of string literals.
    * Should only be called if [isInterpolation] returns false.
    */
   DartString get dartString {
     if (isInterpolation) {
-      throw new SpannableAssertionFailure(
-          this, "Getting dartString on interpolation;");
+      failedAt(this, "Getting dartString on interpolation;");
     }
     if (dartStringCache == null) {
       DartString firstString = first.accept(const GetDartStringVisitor());
       DartString secondString = second.accept(const GetDartStringVisitor());
       if (firstString == null || secondString == null) {
         return null;
       }
       dartStringCache = new DartString.concat(firstString, secondString);
     }
     return dartStringCache;
@@ skipping 1108 matching lines @@
  */
 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;
 }