flutter attach should handle non-singular device gracefully (#19036)

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

@@ -71,6 +71,8 @@ class AttachCommand extends FlutterCommand {
     await _validateArguments();
 
     final Device device = await findTargetDevice();
+    if (device == null)
+      throwToolExit(null);
     final int devicePort = observatoryPort;
     Uri observatoryUri;
     if (devicePort == null) {

packages/flutter_tools/test/commands/attach_test.dart

@@ -4,6 +4,7 @@
 
 import 'dart:async';
 
+import 'package:flutter_tools/src/base/common.dart';
 import 'package:flutter_tools/src/cache.dart';
 import 'package:flutter_tools/src/commands/attach.dart';
 import 'package:flutter_tools/src/device.dart';
@@ -68,6 +69,37 @@ void main() {
 
       verify(portForwarder.forward(devicePort)).called(1);
     });
+
+    testUsingContext('exits when no device connected', () async {
+      final AttachCommand command = new AttachCommand();
+      await expectLater(
+        createTestCommandRunner(command).run(<String>['attach']),
+        throwsA(const isInstanceOf<ToolExit>()),
+      );
+      expect(testLogger.statusText, contains('No connected devices'));
+    });
+
+    testUsingContext('exits when multiple devices connected', () async {
+      Device aDeviceWithId(String id) {
+        final MockAndroidDevice device = new MockAndroidDevice();
+        when(device.name).thenReturn('d$id');
+        when(device.id).thenReturn(id);
+        when(device.isLocalEmulator).thenAnswer((_) async => false);
+        when(device.sdkNameAndVersion).thenAnswer((_) async => 'Android 46');
+        return device;
+      }
+
+      final AttachCommand command = new AttachCommand();
+      testDeviceManager.addDevice(aDeviceWithId('xx1'));
+      testDeviceManager.addDevice(aDeviceWithId('yy2'));
+      await expectLater(
+        createTestCommandRunner(command).run(<String>['attach']),
+        throwsA(const isInstanceOf<ToolExit>()),
+      );
+      expect(testLogger.statusText, contains('More than one device'));
+      expect(testLogger.statusText, contains('xx1'));
+      expect(testLogger.statusText, contains('yy2'));
+    });
   });
 }