Invoke copy_tree.py only once to collect all input file lists.

build/copy_tree.gni

 # 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.
 
 _dart_root = rebase_path("..")
 
 # copy_tree() copies a directory tree rooted at `source` to `dest`, which should
 # be somewhere under $root_out_dir.
 #
 # Optional parameters:
 # exclude - A comma separated list that is passed to shutil.ignore_patterns()
 #           in tools/copy_tree.py.
 template("copy_tree") {
   assert(defined(invoker.source), "copy_tree must define 'source'")
   assert(defined(invoker.dest), "copy_tree must define 'dest'")
+  assert(defined(invoker.inputs), "copy_tree must define 'inputs'")
   source = invoker.source
   dest = invoker.dest
+  inputs = invoker.inputs
   action(target_name) {
     if (defined(invoker.visibility)) {
       visibility = invoker.visibility
     }
 
     deps = []
     if (defined(invoker.deps)) {
       deps += invoker.deps
     }
 
     common_args = [
       "--from",
       rebase_path(source),
       "--to",
       rebase_path(dest),
     ]
     if (defined(invoker.exclude)) {
       common_args += [
         "--exclude",
         invoker.exclude,
       ]
     }
 
-    dry_run_args = common_args + [ "--dry-run" ]
-    input_files = exec_script("$_dart_root/tools/copy_tree.py",
-                              dry_run_args,
-                              "list lines")
-    inputs = input_files
-    relative_files = rebase_path(input_files, rebase_path(source))
+    relative_files = rebase_path(inputs, rebase_path(source))
 
     output_files = []
     foreach(input, relative_files) {
       output_files += [ "$dest/$input" ]
     }
 
     outputs = output_files
     script = "$_dart_root/tools/copy_tree.py"
     args = common_args
   }
 }