Generate typedef node from the element.

tests/compiler/dart2js/backend_dart/dart_backend_test.dart

Index: tests/compiler/dart2js/backend_dart/dart_backend_test.dart
diff --git a/tests/compiler/dart2js/backend_dart/dart_backend_test.dart b/tests/compiler/dart2js/backend_dart/dart_backend_test.dart
index 7392142e3d961a12c121a766d7dd14b77f48887d..ca6bac6e4b778642208af0c58f2e70dd212cb364 100644
--- a/tests/compiler/dart2js/backend_dart/dart_backend_test.dart
+++ b/tests/compiler/dart2js/backend_dart/dart_backend_test.dart
@@ -214,7 +214,7 @@ class A {
   // Maybe typedef should be included in the result too, but it
   // works fine without it.
   testDart2Dart('''
-typedef void foofunc(arg);
+typedef void foofunc(_0);
 main() {
   new A((arg) {});
 }
@@ -669,13 +669,13 @@ main() {
 }
 ''';
   var expectedResult = '''
-typedef void MyFunction<T_B extends num>(T_B arg);
+typedef void MyFunction<T_B extends num>(T_B _0);
 class T {}
 class B<T_B> {}
 class A<T_B> extends B<T_B> {
   T_B f;
 }
-typedef void MyFunction_A<T_B extends num>(T_B arg);
+typedef void MyFunction_A<T_B extends num>(T_B _0);
 class T_A {}
 class B_A<T_B> {}
 class A_A<T_B> extends B_A<T_B> {