Deploy `@immutable` in more places (#9462)

Turns out we have many immutable classes.

Fixes #6892

dev/manual_tests/material_arc.dart

diff --git a/dev/manual_tests/material_arc.dart b/dev/manual_tests/material_arc.dart
index 65d915a..22e6eda 100644
--- a/dev/manual_tests/material_arc.dart
+++ b/dev/manual_tests/material_arc.dart
@@ -52,7 +52,7 @@
   }) : _repaint = repaint, super(repaint: repaint);
 
   final MaterialPointArcTween arc;
-  Animation<double> _repaint;
+  final Animation<double> _repaint;
 
   void drawPoint(Canvas canvas, Offset point, Color color) {
     final Paint paint = new Paint()
@@ -227,7 +227,7 @@
   }) : _repaint = repaint, super(repaint: repaint);
 
   final MaterialRectArcTween arc;
-  Animation<double> _repaint;
+  final Animation<double> _repaint;
 
   void drawPoint(Canvas canvas, Offset p, Color color) {
     final Paint paint = new Paint()