Add a refresh command that rebuilds the snapshot and reloads it on the device

This provides a fast way to iterate on changes to a Flutter app that only
involve updates to Dart code and do not require a full build and install
of the FLX and APK

packages/flutter_tools/lib/executable.dart

@@ -20,6 +20,7 @@ import 'src/commands/ios.dart';
 import 'src/commands/list.dart';
 import 'src/commands/listen.dart';
 import 'src/commands/logs.dart';
+import 'src/commands/refresh.dart';
 import 'src/commands/run_mojo.dart';
 import 'src/commands/start.dart';
 import 'src/commands/stop.dart';
@@ -47,6 +48,7 @@ Future main(List<String> args) async {
     ..addCommand(new ListCommand())
     ..addCommand(new ListenCommand())
     ..addCommand(new LogsCommand())
+    ..addCommand(new RefreshCommand())
     ..addCommand(new RunMojoCommand())
     ..addCommand(new StartCommand())
     ..addCommand(new StopCommand())

packages/flutter_tools/lib/src/android/device_android.dart

@@ -21,6 +21,12 @@ import 'android.dart';
 
 const String _defaultAdbPath = 'adb';
 
+// Path where the FLX bundle will be copied on the device.
+const String _deviceBundlePath = '/data/local/tmp/dev.flx';
+
+// Path where the snapshot will be copied on the device.
+const String _deviceSnapshotPath = '/data/local/tmp/dev_snapshot.bin';
+
 class AndroidDeviceDiscovery extends DeviceDiscovery {
   List<Device> _devices = <Device>[];
 
@@ -268,12 +274,11 @@ class AndroidDevice extends Device {
     if (clearLogs)
       this.clearLogs();
 
-    String deviceTmpPath = '/data/local/tmp/dev.flx';
-    runCheckedSync(adbCommandForDevice(<String>['push', bundlePath, deviceTmpPath]));
+    runCheckedSync(adbCommandForDevice(<String>['push', bundlePath, _deviceBundlePath]));
     List<String> cmd = adbCommandForDevice(<String>[
       'shell', 'am', 'start',
       '-a', 'android.intent.action.RUN',
-      '-d', deviceTmpPath,
+      '-d', _deviceBundlePath,
       '-f', '0x20000000',  // FLAG_ACTIVITY_SINGLE_TOP
     ]);
     if (checked)
@@ -420,6 +425,26 @@ class AndroidDevice extends Device {
   void setConnected(bool value) {
     _connected = value;
   }
+
+  Future<bool> refreshSnapshot(AndroidApk apk, String snapshotPath) async {
+    if (!FileSystemEntity.isFileSync(snapshotPath)) {
+      printError('Cannot find $snapshotPath');
+      return false;
+    }
+
+    runCheckedSync(adbCommandForDevice(<String>['push', snapshotPath, _deviceSnapshotPath]));
+
+    List<String> cmd = adbCommandForDevice(<String>[
+      'shell', 'am', 'start',
+      '-a', 'android.intent.action.RUN',
+      '-d', _deviceBundlePath,
+      '-f', '0x20000000',  // FLAG_ACTIVITY_SINGLE_TOP
+      '--es', 'snapshot', _deviceSnapshotPath,
+      apk.launchActivity,
+    ]);
+    runCheckedSync(cmd);
+    return true;
+  }
 }
 
 /// The [mockAndroid] argument is only to facilitate testing with mocks, so that

packages/flutter_tools/lib/src/commands/refresh.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 'dart:io';
+
+import 'package:path/path.dart' as path;
+
+import '../base/context.dart';
+import '../flx.dart';
+import '../runner/flutter_command.dart';
+
+class RefreshCommand extends FlutterCommand {
+  final String name = 'refresh';
+  final String description = 'Build and deploy the Dart code in a Flutter app (Android only).';
+
+  RefreshCommand() {
+    argParser.addOption('target',
+      abbr: 't',
+      defaultsTo: defaultMainPath,
+      help: 'Target app path / main entry-point file.'
+    );
+  }
+
+  @override
+  Future<int> runInProject() async {
+    printTrace('Downloading toolchain.');
+
+    await Future.wait([
+      downloadToolchain(),
+      downloadApplicationPackagesAndConnectToDevices(),
+    ], eagerError: true);
+
+    if (!devices.android.isConnected()) {
+      printError('No device connected.');
+      return 1;
+    }
+
+    Directory tempDir = await Directory.systemTemp.createTemp('flutter_tools');
+    try {
+      String snapshotPath = path.join(tempDir.path, 'snapshot_blob.bin');
+
+      int result = await toolchain.compiler.compile(
+          mainPath: argResults['target'], snapshotPath: snapshotPath
+      );
+      if (result != 0) {
+        printError('Failed to run the Flutter compiler. Exit code: $result');
+        return result;
+      }
+
+      bool success = await devices.android.refreshSnapshot(
+          applicationPackages.android, snapshotPath
+      );
+      if (!success) {
+        printError('Error refreshing snapshot on ${devices.android.name}.');
+        return 1;
+      }
+
+      return 0;
+    } finally {
+      tempDir.deleteSync(recursive: true);
+    }
+  }
+}