add dart2js.dart in the bin directory.

pkg/compiler/bin/dart2js.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.
+
+/// Entrypoint to run the compiler. This entrypoint is currently only used by
+/// dart2js developers. To run the compiler you now can:
+///   * call `pub get` in the pkg/compiler folder.
+///   * simply call `dart path-to-pkg/compiler/bin/dart2js.dart foo.dart`
+// TODO(sigmund): move `main` here, and change our sdk build tools to just use
+// this entrypoint instead.
+library compiler.bin.dart2js;
+
+export 'package:compiler/src/dart2js.dart';