Fix various nits in VM patch files.

runtime/tools/concatenate_patches.py

 #!/usr/bin/env python
 # Copyright (c) 2016, 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.
 
 from optparse import OptionParser
 
 def writePatch(output_file_name, input_file_names):
   dart_file_names = filter(lambda name: name.endswith('.dart'),
                            input_file_names)
   with open(output_file_name, 'w') as output_file:
+    is_first = True
     for dart_file_name in dart_file_names:
       with open(dart_file_name, 'r') as dart_file:
-        output_file.write(dart_file.read())
+        if is_first:
+          output_file.write(dart_file.read())
+          is_first = False
+        else:
+          for line in dart_file.readlines():
+            if not line.startswith("import "):
+              output_file.write(line)
 
 
 def main():
   parser = OptionParser()
   parser.add_option('--output', action='store', type='string',
                     help='output file path')
   (options, args) = parser.parse_args()
   if not options.output:
     parser.error('missing --output option\n')
   if len(args) == 0:
     parser.error('no input files given\n')
   writePatch(options.output, args)
 
 
 if __name__ == '__main__':
   main()