Improve flutter_tools handling of the device being unplugged while syncing DevFS (#9431)

- [x] Catch SocketErrors and handle them gracefully.
- [x] Print 'Lost connection to device' when the service protocol connection is severed unexpectedly.
- [x] Print 'Application finished' when the application exits otherwise.

After this PR:

```
Launching lib/main.dart on Nexus 7 in debug mode...
Running 'gradle assembleDebug'...                     1.2s
Built build/app/outputs/apk/app-debug.apk (21.7MB).
Syncing files to device...
Application finished.
DevFS sync failed. Lost connection to device: SocketException: OS Error: Connection refused, errno = 111, address = 127.0.0.1, port = 53062
Could not perform initial file synchronization.
```

Fixes #6705

packages/flutter_tools/lib/src/devfs.dart

@@ -220,6 +220,13 @@ class ServiceProtocolDevFSOperations implements DevFSOperations {
   }
 }
 
+class DevFSException implements Exception {
+  DevFSException(this.message, [this.error, this.stackTrace]);
+  final String message;
+  final dynamic error;
+  final StackTrace stackTrace;
+}
+
 class _DevFSHttpWriter {
   _DevFSHttpWriter(this.fsName, VMService serviceProtocol)
       : httpAddress = serviceProtocol.httpAddress;
@@ -279,6 +286,11 @@ class _DevFSHttpWriter {
       await request.addStream(contents);
       final HttpClientResponse response = await request.close();
       await response.drain<Null>();
+    } on SocketException catch (socketException, stackTrace) {
+      // We have one completer and can get up to kMaxInFlight errors.
+      if (!_completer.isCompleted)
+        _completer.completeError(socketException, stackTrace);
+      return;
     } catch (e) {
       if (retry < kMaxRetries) {
         printTrace('Retrying writing "$deviceUri" to DevFS due to error: $e');
@@ -422,8 +434,12 @@ class DevFS {
         try {
           await _httpWriter.write(dirtyEntries,
                                   progressReporter: progressReporter);
-        } catch (e) {
-          printError("Could not update files on device: $e");
+        } on SocketException catch (socketException, stackTrace) {
+          printTrace("DevFS sync failed. Lost connection to device: $socketException");
+          throw new DevFSException('Lost connection to device.', socketException, stackTrace);
+        } catch (exception, stackTrace) {
+          printError("Could not update files on device: $exception");
+          throw new DevFSException('Sync failed', exception, stackTrace);
         }
       } else {
         // Make service protocol requests for each.

packages/flutter_tools/lib/src/resident_runner.dart

@@ -49,6 +49,7 @@ abstract class ResidentRunner {
   final bool usesTerminalUI;
   final bool stayResident;
   final Completer<int> _finished = new Completer<int>();
+  bool _stopped = false;
   String _packagesFilePath;
   String get packagesFilePath => _packagesFilePath;
   String _projectRootPath;
@@ -101,6 +102,7 @@ abstract class ResidentRunner {
   }
 
   Future<Null> stop() async {
+    _stopped = true;
     await stopEchoingDeviceLog();
     await preStop();
     return stopApp();
@@ -259,7 +261,7 @@ abstract class ResidentRunner {
       service.done.then<Null>(
         _serviceProtocolDone,
         onError: _serviceProtocolError
-      ).whenComplete(appFinished);
+      ).whenComplete(_serviceDisconnected);
     }
   }
 
@@ -336,6 +338,18 @@ abstract class ResidentRunner {
     }
   }
 
+  void _serviceDisconnected() {
+    if (_stopped) {
+      // User requested the application exit.
+      return;
+    }
+    if (_finished.isCompleted)
+      return;
+    printStatus('Lost connection to device.');
+    _resetTerminal();
+    _finished.complete(0);
+  }
+
   void appFinished() {
     if (_finished.isCompleted)
       return;

packages/flutter_tools/lib/src/run_hot.dart

@@ -120,7 +120,6 @@ class HotRunner extends ResidentRunner {
     }
     final bool devfsResult = await _updateDevFS();
     if (!devfsResult) {
-      printError('Could not perform initial file synchronization.');
       return 3;
     }
 
@@ -263,10 +262,16 @@ class HotRunner extends ResidentRunner {
     }
     final Status devFSStatus = logger.startProgress('Syncing files to device...',
         expectSlowOperation: true);
-    final int bytes = await _devFS.update(progressReporter: progressReporter,
+    int bytes = 0;
+    try {
+      bytes = await _devFS.update(progressReporter: progressReporter,
           bundle: assetBundle,
           bundleDirty: rebuildBundle,
           fileFilter: _dartDependencies);
+    } on DevFSException {
+      devFSStatus.cancel();
+      return false;
+    }
     devFSStatus.stop();
     if (!hotRunnerConfig.stableDartDependencies) {
       // Clear the set after the sync so they are recomputed next time.
@@ -323,7 +328,7 @@ class HotRunner extends ResidentRunner {
     restartTimer.start();
     final bool updatedDevFS = await _updateDevFS();
     if (!updatedDevFS)
-      return new OperationResult(1, 'Dart Source Error');
+      return new OperationResult(1, 'DevFS Synchronization Failed');
     await _launchFromDevFS(package, mainPath);
     restartTimer.stop();
     printTrace('Restart performed in '
@@ -413,6 +418,8 @@ class HotRunner extends ResidentRunner {
       reassembleTimer = new Stopwatch();
     }
     final bool updatedDevFS = await _updateDevFS();
+    if (!updatedDevFS)
+      return new OperationResult(1, 'DevFS Synchronization Failed');
     if (benchmarkMode) {
       devFSTimer.stop();
       // Record time it took to synchronize to DevFS.