[flutter_tools] Adds some support for '-d all' (#9585)

packages/flutter_tools/lib/src/device.dart

diff --git a/packages/flutter_tools/lib/src/device.dart b/packages/flutter_tools/lib/src/device.dart
index b4e0fbc..7ba2299 100644
--- a/packages/flutter_tools/lib/src/device.dart
+++ b/packages/flutter_tools/lib/src/device.dart
@@ -32,11 +32,26 @@
 
   final List<DeviceDiscovery> _deviceDiscoverers = <DeviceDiscovery>[];
 
-  /// A user-specified device ID.
-  String specifiedDeviceId;
+  String _specifiedDeviceId;
 
+  /// A user-specified device ID.
+  String get specifiedDeviceId {
+    if (_specifiedDeviceId == null || _specifiedDeviceId == 'all')
+      return null;
+    return _specifiedDeviceId;
+  }
+
+  set specifiedDeviceId(String id) {
+    _specifiedDeviceId = id;
+  }
+
+  /// True when the user has specified a single specific device.
   bool get hasSpecifiedDeviceId => specifiedDeviceId != null;
 
+  /// True when the user has specified all devices by setting
+  /// specifiedDeviceId = 'all'.
+  bool get hasSpecifiedAllDevices => _specifiedDeviceId == 'all';
+
   Stream<Device> getDevicesById(String deviceId) async* {
     final Stream<Device> devices = getAllConnectedDevices();
     deviceId = deviceId.toLowerCase();