Make FlutterDevice.generator final (#21555)

Now that Dart 1 support has been eliminated, generator should always be
populated to a kernel compiler instance.

Also moves the constructor to the top of the class, and orders final
fields before non-final fields, as dictated by the style guide.

packages/flutter_tools/lib/src/resident_runner.dart

@@ -29,32 +29,28 @@ import 'run_hot.dart';
 import 'vmservice.dart';
 
 class FlutterDevice {
+  FlutterDevice(this.device, {
+    @required bool trackWidgetCreation,
+    this.dillOutputPath,
+    this.fileSystemRoots,
+    this.fileSystemScheme,
+    ResidentCompiler generator,
+  }) : this.generator = generator ?? new ResidentCompiler(
+         artifacts.getArtifactPath(Artifact.flutterPatchedSdkPath),
+         trackWidgetCreation: trackWidgetCreation,
+         fileSystemRoots: fileSystemRoots, fileSystemScheme: fileSystemScheme
+       );
+
   final Device device;
+  final ResidentCompiler generator;
   List<Uri> observatoryUris;
   List<VMService> vmServices;
   DevFS devFS;
   ApplicationPackage package;
-  ResidentCompiler generator;
   String dillOutputPath;
   List<String> fileSystemRoots;
   String fileSystemScheme;
-
   StreamSubscription<String> _loggingSubscription;
-
-  FlutterDevice(this.device, {
-    @required bool trackWidgetCreation,
-    this.dillOutputPath,
-    this.fileSystemRoots,
-    this.fileSystemScheme,
-    ResidentCompiler generator,
-  }) {
-    this.generator = generator ?? new ResidentCompiler(
-      artifacts.getArtifactPath(Artifact.flutterPatchedSdkPath),
-      trackWidgetCreation: trackWidgetCreation,
-      fileSystemRoots: fileSystemRoots, fileSystemScheme: fileSystemScheme
-    );
-  }
-
   String viewFilter;
 
   /// If the [reloadSources] parameter is not null the 'reloadSources' service