Set an empty sourcepath when compiling java.

build/android/gyp/javac.py

 #!/usr/bin/env python
 #
 # Copyright 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import optparse
 import os
 import shutil
 import re
@@ skipping 304 matching lines @@
   javac_cmd = ['javac']
   if options.use_errorprone_path:
     javac_cmd = [options.use_errorprone_path] + ERRORPRONE_OPTIONS
 
   javac_cmd.extend((
       '-g',
       # Chromium only allows UTF8 source files.  Being explicit avoids
       # javac pulling a default encoding from the user's environment.
       '-encoding', 'UTF-8',
       '-classpath', ':'.join(compile_classpath),
+      # Prevent compiler from compiling .java files not listed as inputs.
+      # See: http://blog.ltgt.net/most-build-tools-misuse-javac/
+      '-sourcepath', ''
   ))
 
   if options.bootclasspath:
     javac_cmd.extend([
         '-bootclasspath', ':'.join(options.bootclasspath),
         '-source', '1.7',
         '-target', '1.7',
         ])
 
   if options.chromium_code:
@@ skipping 36 matching lines @@
       options,
       input_paths=input_paths,
       input_strings=javac_cmd,
       output_paths=output_paths,
       force=force,
       pass_changes=True)
 
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv[1:]))