[infra] Fix copy_tree.py to avoid extra rebuilding in Fuchsia

tools/copy_tree.py

 #!/usr/bin/env python
 # Copyright (c) 2017, 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 argparse
 import os
 import re
 import shutil
 import sys
 
 def ParseArgs(args):
   args = args[1:]
   parser = argparse.ArgumentParser(
       description='A script to copy a file tree somewhere')
 
+  parser.add_argument('--dry-run', '-d',
+      dest='dryrun',
+      default=False,
+      action='store_true',
+      help='Print the paths of the source files, but do not copy anything.')
   parser.add_argument('--exclude_patterns', '-e',
       type=str,
       help='Patterns to exclude [passed to shutil.copytree]')
   parser.add_argument('--from', '-f',
       dest="copy_from",
       type=str,
       required=True,
-      help='Source tree root')
+      help='Source directory')
   parser.add_argument('--to', '-t',
       type=str,
       required=True,
-      help='Destination')
+      help='Destination directory')
 
   return parser.parse_args(args)
 
 
 def ValidateArgs(args):
   if not os.path.isdir(args.copy_from):
     print "--from argument must refer to a directory"
     return False
   return True
 
 
+def CopyTree(src, dst, dryrun=False, symlinks=False, ignore=None):
+  names = os.listdir(src)
+  if ignore is not None:
+    ignored_names = ignore(src, names)
+  else:
+    ignored_names = set()
+
+  if not dryrun:
+    os.makedirs(dst)
+  errors = []
+  for name in names:
+    if name in ignored_names:
+      continue
+    srcname = os.path.join(src, name)
+    dstname = os.path.join(dst, name)
+    try:
+      if os.path.isdir(srcname):
+        CopyTree(srcname, dstname, dryrun, symlinks, ignore)
+      else:
+        if dryrun:
+          print srcname
+        else:
+          shutil.copy(srcname, dstname)
+    except (IOError, os.error) as why:
+      errors.append((srcname, dstname, str(why)))
+    # catch the Error from the recursive CopyTree so that we can
+    # continue with other files
+    except Error as err:
+      errors.extend(err.args[0])
+  try:
+    if not dryrun:
+      shutil.copystat(src, dst)
+  except WindowsError:
+    # can't copy file access times on Windows
+    pass
+  except OSError as why:
+    errors.extend((src, dst, str(why)))
+  if errors:
+    raise Error(errors)
+
+
 def Main(argv):
   args = ParseArgs(argv)
   if not ValidateArgs(args):
     return -1
+
   if os.path.exists(args.to):
     shutil.rmtree(args.to)
   if args.exclude_patterns == None:
-    shutil.copytree(args.copy_from, args.to)
+    CopyTree(args.copy_from, args.to, dryrun=args.dryrun)
   else:
     patterns = args.exclude_patterns.split(',')
-    shutil.copytree(args.copy_from, args.to,
-                    ignore=shutil.ignore_patterns(tuple(patterns)))
+    CopyTree(args.copy_from, args.to, dryrun=args.dryrun,
+             ignore=shutil.ignore_patterns(*patterns))
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(Main(sys.argv))