Add typedef AST node boilerplate.

pkg/kernel/lib/transformations/reify/transformation/remove_generics.dart

Index: pkg/kernel/lib/transformations/reify/transformation/remove_generics.dart
diff --git a/pkg/kernel/lib/transformations/reify/transformation/remove_generics.dart b/pkg/kernel/lib/transformations/reify/transformation/remove_generics.dart
index 1a103f6abe97f365d22443d63bb31131350e6696..439d557498ca5d447a99cc789f1256851a686484 100644
--- a/pkg/kernel/lib/transformations/reify/transformation/remove_generics.dart
+++ b/pkg/kernel/lib/transformations/reify/transformation/remove_generics.dart
@@ -60,6 +60,11 @@ class Erasure extends Transformer with DartTypeVisitor<DartType> {
   }
 
   @override
+  TypedefType visitTypedefType(TypedefType type) {
+    throw 'Typedef types not implemented in erasure';
+  }
+
+  @override
   Supertype visitSupertype(Supertype type) {
     if (removeTypeParameters(type.classNode)) {
       return new Supertype(type.classNode, const <DartType>[]);