Revert "Add typedef AST node boilerplate."

pkg/kernel/lib/transformations/treeshaker.dart

Index: pkg/kernel/lib/transformations/treeshaker.dart
diff --git a/pkg/kernel/lib/transformations/treeshaker.dart b/pkg/kernel/lib/transformations/treeshaker.dart
index 8375f7604ba9ee01d616785f5b093a950ccacf9f..1d89c66dfff675a8cc5302f8fb566dbc7d401b7f 100644
--- a/pkg/kernel/lib/transformations/treeshaker.dart
+++ b/pkg/kernel/lib/transformations/treeshaker.dart
@@ -1056,10 +1056,6 @@ class _ExternalTypeVisitor extends DartTypeVisitor {
     }
   }
 
-  visitTypedefType(TypedefType node) {
-    throw 'TypedefType is not implemented in tree shaker';
-  }
-
   visitFunctionType(FunctionType node) {
     visit(node.returnType);
     for (int i = 0; i < node.positionalParameters.length; ++i) {