Step 2 (#57751)

packages/flutter/lib/src/material/dialog.dart

@@ -58,7 +58,7 @@ class Dialog extends StatelessWidget {
     this.child,
     bool useMaterialBorderRadius,
   }) : assert(clipBehavior != null),
-       useMaterialBorderRadius = useMaterialBorderRadius ?? false,
+       useMaterialBorderRadius = useMaterialBorderRadius ?? true,
        super(key: key);
 
   /// {@template flutter.material.dialog.backgroundColor}
@@ -282,7 +282,7 @@ class AlertDialog extends StatelessWidget {
     bool useMaterialBorderRadius,
   }) : assert(contentPadding != null),
        assert(clipBehavior != null),
-       useMaterialBorderRadius = useMaterialBorderRadius ?? false,
+       useMaterialBorderRadius = useMaterialBorderRadius ?? true,
        super(key: key);
 
   /// The (optional) title of the dialog is displayed in a large font at the top
@@ -754,7 +754,7 @@ class SimpleDialog extends StatelessWidget {
     bool useMaterialBorderRadius,
   }) : assert(titlePadding != null),
        assert(contentPadding != null),
-       useMaterialBorderRadius = useMaterialBorderRadius ?? false,
+       useMaterialBorderRadius = useMaterialBorderRadius ?? true,
        super(key: key);
 
   /// The (optional) title of the dialog is displayed in a large font at the top

packages/flutter/lib/src/material/time_picker.dart

@@ -1507,7 +1507,7 @@ class _TimePickerDialog extends StatefulWidget {
     @required this.initialTime,
     bool useMaterialBorderRadius,
   }) : assert(initialTime != null),
-       useMaterialBorderRadius = useMaterialBorderRadius ?? false,
+       useMaterialBorderRadius = useMaterialBorderRadius ?? true,
        super(key: key);
 
   /// The time initially selected when the dialog is shown.
@@ -1842,7 +1842,7 @@ Future<TimeOfDay> showTimePicker({
 
   final Widget dialog = _TimePickerDialog(
     initialTime: initialTime,
-    useMaterialBorderRadius: useMaterialBorderRadius ?? false,
+    useMaterialBorderRadius: useMaterialBorderRadius ?? true,
   );
   return await showDialog<TimeOfDay>(
     context: context,

packages/flutter/test/material/dialog_test.dart

@@ -94,7 +94,6 @@ void main() {
       title: Text('Title'),
       content: Text('Y'),
       actions: <Widget>[ ],
-      useMaterialBorderRadius: true,
     );
     await tester.pumpWidget(_buildAppWithDialog(dialog, theme: ThemeData(brightness: Brightness.dark)));
 
@@ -190,7 +189,6 @@ void main() {
     const AlertDialog dialog = AlertDialog(
       actions: <Widget>[ ],
       shape: null,
-      useMaterialBorderRadius: true,
     );
     await tester.pumpWidget(_buildAppWithDialog(dialog));