Revert "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 439d557498ca5d447a99cc789f1256851a686484..1a103f6abe97f365d22443d63bb31131350e6696 100644
--- a/pkg/kernel/lib/transformations/reify/transformation/remove_generics.dart
+++ b/pkg/kernel/lib/transformations/reify/transformation/remove_generics.dart
@@ -60,11 +60,6 @@ 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>[]);