Handle synthetic nodes use to throw exceptions

pkg/compiler/lib/src/kernel/kernel.dart

 // Copyright (c) 2016, 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.md file.
 
 import 'dart:async';
 import 'dart:collection' show Queue;
 
 import 'package:kernel/ast.dart' as ir;
 import 'package:kernel/verifier.dart' show CheckParentPointers;
 
@@ skipping 56 matching lines @@
       <TypeVariableElement, ir.TypeParameter>{};
 
   final Set<ir.TreeNode> checkedNodes = new Set<ir.TreeNode>();
 
   final Map<LibraryElement, Map<String, int>> mixinApplicationNamesByLibrary =
       <LibraryElement, Map<String, int>>{};
 
   final Map<ir.Node, Element> nodeToElement = <ir.Node, Element>{};
   final Map<ir.Node, Node> nodeToAst = <ir.Node, Node>{};
   final Map<ir.Node, Node> nodeToAstOperator = <ir.Node, Node>{};
+  // Synthetic nodes are nodes we generated that do not correspond to
+  // [ast.Node]s. A node should be in one of nodeToAst or syntheticNodes but not
+  // both.
+  final Set<ir.Node> syntheticNodes = new Set<ir.Node>();
 
   /// FIFO queue of work that needs to be completed before the returned AST
   /// nodes are correct.
   final Queue<WorkItem> workQueue = new Queue<WorkItem>();
 
   Kernel(this.compiler);
 
   void addWork(Element element, WorkAction action) {
     workQueue.addLast(new WorkItem(element, action));
   }
@@ skipping 668 matching lines @@
 }
 
 class ConstructorTarget {
   final ConstructorElement element;
   final DartType type;
 
   ConstructorTarget(this.element, this.type);
 
   String toString() => "ConstructorTarget($element, $type)";
 }