Support snapshots in sky_shell (again)

sky/tools/packager/main.cc

 // Copyright 2015 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.
 
 #include <iostream>
 
 #include "base/at_exit.h"
 #include "base/basictypes.h"
 #include "base/command_line.h"
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/logging.h"
 #include "base/process/memory.h"
 #include "dart/runtime/include/dart_api.h"
 #include "sky/tools/packager/loader.h"
 #include "sky/tools/packager/logging.h"
 #include "sky/tools/packager/scope.h"
 #include "sky/tools/packager/switches.h"
 #include "sky/tools/packager/vm.h"
 
+void Usage() {
+  std::cerr << "Usage: sky_packager"
+            << " --" << switches::kPackageRoot << " --" << switches::kSnapshot
+            << " <sky-app>" << std::endl;
+}
+
 void WriteSnapshot(base::FilePath path) {
   uint8_t* buffer;
   intptr_t size;
   CHECK(!LogIfError(Dart_CreateScriptSnapshot(&buffer, &size)));
 
   CHECK_EQ(base::WriteFile(path, reinterpret_cast<const char*>(buffer), size),
            size);
 
   std::cout << "Successfully wrote snapshot to " << path.LossyDisplayName()
             << " (" << size << " bytes)." << std::endl;
 }
 
 int main(int argc, const char* argv[]) {
   base::AtExitManager exit_manager;
   base::EnableTerminationOnHeapCorruption();
   base::CommandLine::Init(argc, argv);
 
+  const base::CommandLine& command_line =
+      *base::CommandLine::ForCurrentProcess();
+
+  if (command_line.HasSwitch(switches::kHelp) ||
+      command_line.GetArgs().empty()) {
+    Usage();
+    return 0;
+  }
+
   InitDartVM();
   Dart_Isolate isolate = CreateDartIsolate();
   CHECK(isolate);
 
-  const base::CommandLine& command_line =
-      *base::CommandLine::ForCurrentProcess();
-
   DartIsolateScope scope(isolate);
   DartApiScope api_scope;
 
   auto args = command_line.GetArgs();
   CHECK(args.size() == 1);
   LoadScript(args[0]);
 
   CHECK(!LogIfError(Dart_FinalizeLoading(true)));
 
-  CHECK(command_line.HasSwitch(kSnapshot)) << "Need --snapshot";
-  WriteSnapshot(command_line.GetSwitchValuePath(kSnapshot));
+  CHECK(command_line.HasSwitch(switches::kSnapshot)) << "Need --snapshot";
+  WriteSnapshot(command_line.GetSwitchValuePath(switches::kSnapshot));
 
   return 0;
 }