Do not mark type variables as needing a box inside try; they are immutable.

tests/language/regress_23650_test.dart

 // Copyright (c) 2015, 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.
 
-import "dart:async";
+// Test that type variables in try-catch work.
+
 import "package:expect/expect.dart";
 
-main() async {
-  var error = "no error";
-  try {
+class C<T> {
+  C.foo();
+  factory C() {
     try {
-      await new Future.error("error");
-    } on MissingType catch(e) {
+      return new C<T>.foo();
+    } finally {
     }
-  } catch (e) {
-    error = e;
   }
-  Expect.isTrue(error is TypeError);
 }
+
+main() {
+  var c = new C<int>();
+  Expect.isTrue(c is C<int>);
+  Expect.isFalse(c is C<String>);
+}