Fix for issue #5227: do not declare type variables in the closure analyzer when they are not needed.

tests/language/closure_in_constructor_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.
+
+class A<T> {
+  var closure;
+
+  factory A.factory() => new A(() => new List<T>());
+
+  A([this.closure]) {
+    if (closure == null) {
+      closure = () => new List<T>();
+    }
+  }
+}
+
+main() {
+  Expect.isTrue((new A.factory()).closure() is List);
+  Expect.isTrue((new A()).closure() is List);
+}