Add typedef AST node boilerplate.

pkg/kernel/lib/canonical_name.dart

Index: pkg/kernel/lib/canonical_name.dart
diff --git a/pkg/kernel/lib/canonical_name.dart b/pkg/kernel/lib/canonical_name.dart
index f4d2deffb6d12a729ac3758fb1d1d2867148dec6..daddfc9e502a2a54db7edc52efcd2349b1ab0302 100644
--- a/pkg/kernel/lib/canonical_name.dart
+++ b/pkg/kernel/lib/canonical_name.dart
@@ -104,6 +104,10 @@ class CanonicalName {
         .getChildFromQualifiedName(member.name);
   }
 
+  CanonicalName getChildFromTypedef(Typedef typedef_) {
+    return getChild('@typedefs').getChild(typedef_.name);
+  }
+
   void bindTo(Reference target) {
     if (reference == target) return;
     if (reference != null) {