Add --bazel-paths option to dart2js entrypoint.

pkg/compiler/lib/src/commandline_options.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.
 
 library dart2js.cmdline.options;
 
 /// Commandline flags used in `dart2js.dart` and/or `apiimpl.dart`.
 class Flags {
   static const String allowMockCompilation = '--allow-mock-compilation';
   static const String allowNativeExtensions = '--allow-native-extensions';
@@ skipping 55 matching lines @@
   static const String resolveOnly = '--resolve-only';
   static const String initializingFormalAccess = '--initializing-formal-access';
 }
 
 class Option {
   static const String showPackageWarnings =
       '${Flags.showPackageWarnings}|${Flags.showPackageWarnings}=.*';
 
   // Experimental options.
   static const String resolutionInput = '--resolution-input=.+';
+  static const String bazelPaths = '--bazel-paths=.+';
 }