Split the snapshot generation into a separate action (flutter/engine#3168)

This is to ensure that the depfile generated by the snapshotter has
the correct target.

engine/src/flutter/build/flutter_app.gni

@@ -63,7 +63,9 @@ template("flutter_app") {
 
   main_dart = invoker.main_dart
 
-  action(target_name) {
+  flutter_snapshot_name = target_name + "_snapshot"
+
+  action(flutter_snapshot_name) {
     depfile = depfile_path
 
     inputs = [
@@ -71,7 +73,6 @@ template("flutter_app") {
     ]
 
     outputs = [
-      bundle_path,
       snapshot_path,
     ]
 
@@ -79,6 +80,45 @@ template("flutter_app") {
       sources = invoker.sources
     }
 
+    script = "//flutter/build/snapshot.py"
+
+    args = [
+      "--snapshotter-path",
+      rebase_path(flutter_snapshot),
+      "--app-dir",
+      rebase_path("."),
+      "--main-dart",
+      rebase_path(main_dart),
+      "--packages",
+      rebase_path(dot_packages),
+      "--snapshot",
+      rebase_path(snapshot_path),
+      "--depfile",
+      rebase_path(depfile_path),
+      "--build-output",
+      rebase_path(snapshot_path, root_build_dir),
+
+    ]
+
+    deps = [
+      ":$dart_package_name",
+      flutter_snapshot_label,
+    ]
+
+    if (defined(invoker.deps)) {
+      deps += invoker.deps
+    }
+  }
+
+  action(target_name) {
+    inputs = [
+      snapshot_path,
+    ]
+
+    outputs = [
+      bundle_path,
+    ]
+
     script = "//flutter/build/package.py"
 
     args = [
@@ -92,33 +132,22 @@ template("flutter_app") {
       rebase_path(flutter_tools_packages),
       "--flutter-tools-main",
       rebase_path(flutter_tools_main),
-      "--snapshotter-path",
-      rebase_path(flutter_snapshot),
       "--working-dir",
       rebase_path("$target_gen_dir/build"),
       "--app-dir",
       rebase_path("."),
-      "--main-dart",
-      rebase_path(main_dart),
       "--packages",
       rebase_path(dot_packages),
       "--output-file",
       rebase_path(bundle_path),
       "--snapshot",
       rebase_path(snapshot_path),
-      "--depfile",
-      rebase_path(depfile_path),
     ]
 
     deps = [
-      ":$dart_package_name",
+      ":$flutter_snapshot_name",
       dart_binary_label,
-      flutter_snapshot_label,
       flutter_tools_label,
     ]
-
-    if (defined(invoker.deps)) {
-      deps += invoker.deps
-    }
   }
 }

engine/src/flutter/build/package.py

@@ -22,20 +22,14 @@ def main():
                       help='The package map for the Flutter tool')
   parser.add_argument('--flutter-tools-main', type=str, required=True,
                       help='The main.dart file for the Flutter tool')
-  parser.add_argument('--snapshotter-path', type=str, required=True,
-                      help='The Flutter snapshotter')
   parser.add_argument('--working-dir', type=str, required=True,
                       help='The directory where to put intermediate files')
   parser.add_argument('--app-dir', type=str, required=True,
                       help='The root of the app')
-  parser.add_argument('--main-dart', type=str, required=True,
-                      help='The main.dart file to use')
   parser.add_argument('--packages', type=str, required=True,
                       help='The package map to use')
   parser.add_argument('--snapshot', type=str, required=True,
                       help='Path to application snapshot')
-  parser.add_argument('--depfile', type=str, required=True,
-                      help='Where to output dependency information')
   parser.add_argument('--output-file', type=str, required=True,
                       help='Where to output application bundle')
 
@@ -49,12 +43,9 @@ def main():
     args.dart,
     '--packages=%s' % args.flutter_tools_packages,
     args.flutter_tools_main,
-    '--snapshotter-path=%s' % args.snapshotter_path,
     '--working-dir=%s' % args.working_dir,
-    '--target=%s' % args.main_dart,
     '--packages=%s' % args.packages,
     '--snapshot=%s' % args.snapshot,
-    '--depfile=%s' % args.depfile,
     '--output-file=%s' % args.output_file,
     '--header=#!mojo mojo:flutter_content_handler',
   ], env=env, cwd=args.app_dir)

engine/src/flutter/build/snapshot.py

@@ -0,0 +1,45 @@
+#!/usr/bin/env python
+# Copyright 2016 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 argparse
+import subprocess
+import os
+import sys
+
+
+def main():
+  parser = argparse.ArgumentParser(description='Snapshot a Flutter application')
+
+  parser.add_argument('--snapshotter-path', type=str, required=True,
+                      help='The Flutter snapshotter')
+  parser.add_argument('--app-dir', type=str, required=True,
+                      help='The root of the app')
+  parser.add_argument('--main-dart', type=str, required=True,
+                      help='The main.dart file to use')
+  parser.add_argument('--packages', type=str, required=True,
+                      help='The package map to use')
+  parser.add_argument('--snapshot', type=str, required=True,
+                      help='Path to application snapshot')
+  parser.add_argument('--depfile', type=str, required=True,
+                      help='Where to output dependency information')
+  parser.add_argument('--build-output', type=str, required=True,
+                      help='Target name used in the depfile')
+
+  args = parser.parse_args()
+
+  result = subprocess.call([
+    args.snapshotter_path,
+    '--packages=%s' % args.packages,
+    '--snapshot=%s' % args.snapshot,
+    '--depfile=%s' % args.depfile,
+    '--build-output=%s' % args.build_output,
+    args.main_dart,
+  ], cwd=args.app_dir)
+
+  return result
+
+
+if __name__ == '__main__':
+  sys.exit(main())