Make PollingDeviceDiscovery start the initial poll faster. (#130755)

This will speed up the initial population of the device list.

packages/flutter_tools/lib/src/device.dart

@@ -504,12 +504,13 @@ abstract class PollingDeviceDiscovery extends DeviceDiscovery {
     if (_timer == null) {
       deviceNotifier ??= ItemListNotifier<Device>();
       // Make initial population the default, fast polling timeout.
-      _timer = _initTimer(null);
+      _timer = _initTimer(null, initialCall: true);
     }
   }
 
-  Timer _initTimer(Duration? pollingTimeout) {
-    return Timer(_pollingInterval, () async {
+  Timer _initTimer(Duration? pollingTimeout, {bool initialCall = false}) {
+    // Poll for devices immediately on the initial call for faster initial population.
+    return Timer(initialCall ? Duration.zero : _pollingInterval, () async {
       try {
         final List<Device> devices = await pollingGetDevices(timeout: pollingTimeout);
         deviceNotifier!.updateWithNewList(devices);

packages/flutter_tools/test/general.shard/device_test.dart

@@ -234,7 +234,6 @@ void main() {
     FakeAsync().run((FakeAsync time) {
       final FakePollingDeviceDiscovery pollingDeviceDiscovery = FakePollingDeviceDiscovery();
       pollingDeviceDiscovery.startPolling();
-      time.elapse(const Duration(milliseconds: 4001));
 
       // First check should use the default polling timeout
       // to quickly populate the list.

packages/flutter_tools/test/general.shard/proxied_devices/proxied_devices_test.dart

@@ -397,10 +397,7 @@ void main() {
       expect(devicesAdded[0].id, fakeDevice['id']);
       expect(devicesAdded[1].id, fakeDevice2['id']);
     });
-    // Explicit timeout is needed because the default timeout is 2s, but `startPolling` waits for
-    // 4s before making its first poll.
-    // TODO(chingjun): Remove the timeout.
-  }, timeout: const Timeout(Duration(seconds: 6)));
+  });
 
   group('ProxiedDartDevelopmentService', () {
     testWithoutContext('forwards start and shutdown to remote', () async {