prefer_is_empty and prefer_is_not_empty (#8474)

diff --git a/packages/flutter_tools/lib/src/devfs.dart b/packages/flutter_tools/lib/src/devfs.dart
index 468dca2..e1f0236 100644
--- a/packages/flutter_tools/lib/src/devfs.dart
+++ b/packages/flutter_tools/lib/src/devfs.dart

@@ -247,7 +247,7 @@
 
   void _scheduleWrites(DevFSProgressReporter progressReporter) {
     while (_inFlight < kMaxInFlight) {
-      if (_outstanding.length == 0) {
+      if (_outstanding.isEmpty) {
         // Finished.
         break;
       }
@@ -289,7 +289,7 @@
       progressReporter(_done, _max);
     }
     _inFlight--;
-    if ((_outstanding.length == 0) && (_inFlight == 0)) {
+    if ((_outstanding.isEmpty) && (_inFlight == 0)) {
       _completer.complete(null);
     } else {
       _scheduleWrites(progressReporter);
@@ -411,7 +411,7 @@
           assetPathsToEvict.add(archivePath);
       }
     });
-    if (dirtyEntries.length > 0) {
+    if (dirtyEntries.isNotEmpty) {
       printTrace('Updating files');
       if (_httpWriter != null) {
         try {

diff --git a/packages/flutter_tools/lib/src/ios/devices.dart b/packages/flutter_tools/lib/src/ios/devices.dart
index bae97e6..343956d 100644
--- a/packages/flutter_tools/lib/src/ios/devices.dart
+++ b/packages/flutter_tools/lib/src/ios/devices.dart

@@ -251,7 +251,7 @@
       '--justlaunch',
     ];
 
-    if (launchArguments.length > 0) {
+    if (launchArguments.isNotEmpty) {
       launchCommand.add('--args');
       launchCommand.add('${launchArguments.join(" ")}');
     }

diff --git a/packages/flutter_tools/test/devfs_test.dart b/packages/flutter_tools/test/devfs_test.dart
index 1b90421..c802a9b 100644
--- a/packages/flutter_tools/test/devfs_test.dart
+++ b/packages/flutter_tools/test/devfs_test.dart

@@ -329,7 +329,7 @@
 }
 
 void _cleanupTempDirs() {
-  while (_tempDirs.length > 0) {
+  while (_tempDirs.isNotEmpty) {
     _tempDirs.removeLast().deleteSync(recursive: true);
   }
 }