Mix in [TreeElementMixin] only on nodes that need it.

tests/compiler/dart2js/closure_codegen_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 // Test that parameters keep their names in the output.
 
 import 'dart:async';
 import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
 import 'compiler_helper.dart';
 
@@ skipping 15 matching lines @@
 main() {
   var o = null;
   o(1, 2);
 }
 """;
 
 const String TEST_BAILOUT = r"""
 class A {
   var x;
   foo(_) { // make sure only g has no arguments
-    var f = function g() { return 499;  };
+    var f = () { return 499;  };
     return 499 + x + f();
   }
 }
 
 main() { new A().foo(1); }
 """;
 
 Future closureInvocation(bool minify, String prefix) {
   return Future.wait([
     compile(TEST_INVOCATION0, minify: minify, check: (String generated) {
@@ skipping 19 matching lines @@
 }
 
 main() {
   asyncTest(() => Future.wait([
     closureInvocation(false, "call"),
     closureInvocation(true, ""),
     closureBailout(false, "call"),
     closureBailout(true, ""),
   ]));
 }