add explicit const keywords (#23652)

dev/devicelab/bin/tasks/named_isolates_test.dart

@@ -32,7 +32,7 @@ void main() {
     section('Verify all the debug isolate names are set');
     runProcess.stdin.write('l');
     await Future.wait<dynamic>(<Future<dynamic>>[firstNameFound.future, secondNameFound.future])
-                .timeout(Duration(seconds: 1), onTimeout: () => throw 'Isolate names not found.');
+                .timeout(const Duration(seconds: 1), onTimeout: () => throw 'Isolate names not found.');
     await _quitRunner(runProcess);
 
     section('Attach to the second debug isolate');

examples/flutter_gallery/lib/demo/cupertino/cupertino_picker_demo.dart

@@ -21,7 +21,7 @@ class CupertinoPickerDemo extends StatefulWidget {
 class _CupertinoPickerDemoState extends State<CupertinoPickerDemo> {
   int _selectedColorIndex = 0;
 
-  Duration timer = Duration();
+  Duration timer = const Duration();
 
   // Value that is shown in the date picker in date mode.
   DateTime date = DateTime.now();
@@ -276,4 +276,4 @@ class _CupertinoPickerDemoState extends State<CupertinoPickerDemo> {
       ),
     );
   }
-}
\ No newline at end of file
+}

packages/flutter/test/widgets/gesture_detector_test.dart

@@ -340,7 +340,7 @@ void main() {
       await gesture.up();
     }
 
-    await longPress(kLongPressTimeout + Duration(seconds: 1)); // To make sure the time for long press has occured
+    await longPress(kLongPressTimeout + const Duration(seconds: 1)); // To make sure the time for long press has occurred
     expect(longPressUp, 1);
   });
 }

packages/flutter_tools/test/devfs_test.dart

@@ -71,7 +71,7 @@ void main() {
       file.parent.createSync(recursive: true);
       file.writeAsBytesSync(<int>[1, 2, 3]);
 
-      final DateTime fiveSecondsAgo = DateTime.now().subtract(Duration(seconds:5));
+      final DateTime fiveSecondsAgo = DateTime.now().subtract(const Duration(seconds:5));
       expect(content.isModifiedAfter(fiveSecondsAgo), isTrue);
       expect(content.isModifiedAfter(fiveSecondsAgo), isTrue);
       expect(content.isModifiedAfter(null), isTrue);