Throw an exception when invariant fails and report nice diagnostic.

dart/lib/compiler/implementation/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.
 
 part of tree;
 
 abstract class Visitor<R> {
   const Visitor();
 
   abstract R visitNode(Node node);
@@ skipping 75 matching lines @@
   if (first != null) {
     token = first.getBeginToken();
   }
   if (token == null && second != null) {
     // [token] might be null even when [first] is not, e.g. for empty Modifiers.
     token = second.getBeginToken();
   }
   return token;
 }
 
-class NodeAssertionFailure implements Exception {
-  final Node node;
-  final String message;
-  NodeAssertionFailure(this.node, this.message);
-}
-
 /**
  * A node in a syntax tree.
  *
  * The abstract part of "abstract syntax tree" is invalidated when
  * supporting tools such as code formatting. These tools need concrete
  * syntax such as parentheses and no constant folding.
  *
  * We support these tools by storing additional references back to the
  * token stream. These references are stored in fields ending with
  * "Token".
@@ skipping 1185 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 NodeAssertionFailure(this,
-                                     "Getting dartString on interpolation;");
+      throw new SpannableAssertionFailure(
+          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 698 matching lines @@
  * argument).
  *
  * TODO(ahe): This method is controversial, the team needs to discuss
  * if top-level methods are acceptable and what naming conventions to
  * use.
  */
 initializerDo(Node node, f(Node node)) {
   SendSet send = node.asSendSet();
   if (send != null) return f(send.arguments.head);
 }