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

build/copy_tree.gni

Index: build/copy_tree.gni
diff --git a/build/copy_tree.gni b/build/copy_tree.gni
index b3a4c0c298500121f2cd111b9aa6f287c0315876..a6b72b91a1b1e71b16e13cf9854b1a1328dcef76 100644
--- a/build/copy_tree.gni
+++ b/build/copy_tree.gni
@@ -13,8 +13,10 @@ _dart_root = rebase_path("..")
 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
@@ -38,12 +40,7 @@ template("copy_tree") {
       ]
     }
 
-    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) {