Make fields of BoundClosure private to not expose implementation details.

tests/language/closure_internals_test.dart

+// Copyright (c) 2013, 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 "package:expect/expect.dart";
+
+class C {
+  foo() => 123;
+}
+
+main() {
+  var f = new C().foo;
+  Expect.throws(() => f.target, (e) => e is NoSuchMethodError);
+  Expect.throws(() => f.self, (e) => e is NoSuchMethodError);
+  Expect.throws(() => f.receiver, (e) => e is NoSuchMethodError);
+}