Add `flutter precache` (#3207)

This command explicitly populates the flutter tool's cache of binary artifacts.
Also, teach `flutter create` to update the cache in case its the first command
that a user runs.

packages/flutter_tools/lib/executable.dart

@@ -23,6 +23,7 @@ import 'src/commands/drive.dart';
 import 'src/commands/install.dart';
 import 'src/commands/listen.dart';
 import 'src/commands/logs.dart';
+import 'src/commands/precache.dart';
 import 'src/commands/refresh.dart';
 import 'src/commands/run.dart';
 import 'src/commands/run_mojo.dart';
@@ -63,6 +64,7 @@ Future<Null> main(List<String> args) async {
     ..addCommand(new InstallCommand())
     ..addCommand(new ListenCommand())
     ..addCommand(new LogsCommand())
+    ..addCommand(new PrecacheCommand())
     ..addCommand(new RefreshCommand())
     ..addCommand(new RunCommand())
     ..addCommand(new RunMojoCommand(hidden: !verboseHelp))

packages/flutter_tools/lib/src/commands/create.dart

@@ -11,6 +11,7 @@ import 'package:path/path.dart' as path;
 import '../android/android.dart' as android;
 import '../artifacts.dart';
 import '../base/utils.dart';
+import '../cache.dart';
 import '../dart/pub.dart';
 import '../globals.dart';
 import '../template.dart';
@@ -66,6 +67,8 @@ class CreateCommand extends Command {
       return 2;
     }
 
+    await Cache.instance.updateAll();
+
     String flutterRoot = path.absolute(ArtifactStore.flutterRoot);
 
     String flutterPackagesDirectory = path.join(flutterRoot, 'packages');

packages/flutter_tools/lib/src/commands/precache.dart

+// 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 'dart:async';
+
+import 'package:args/command_runner.dart';
+
+import '../cache.dart';
+
+class PrecacheCommand extends Command {
+  @override
+  final String name = 'precache';
+
+  @override
+  final String description = 'Populates the Flutter tool\'s cache of binary artifacts.';
+
+  @override
+  Future<int> run() async {
+    await Cache.instance.updateAll();
+    return 0;
+  }
+}