Handle synthetic nodes use to throw exceptions

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

Index: pkg/compiler/lib/src/kernel/kernel.dart
diff --git a/pkg/compiler/lib/src/kernel/kernel.dart b/pkg/compiler/lib/src/kernel/kernel.dart
index 12105012a6fb4928e92c1c6c5d0956c511af5586..c1e777d45ba53b55f7dbf17b8676ca63ca9d0513 100644
--- a/pkg/compiler/lib/src/kernel/kernel.dart
+++ b/pkg/compiler/lib/src/kernel/kernel.dart
@@ -74,6 +74,10 @@ class Kernel {
   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.