enable lint avoid_field_initializers_in_const_classes (#18415)

analysis_options.yaml

@@ -57,7 +57,7 @@ linter:
     - avoid_classes_with_only_static_members
     # - avoid_double_and_int_checks # only useful when targeting JS runtime
     - avoid_empty_else
-    # - avoid_field_initializers_in_const_classes # not yet tested
+    - avoid_field_initializers_in_const_classes
     - avoid_function_literals_in_foreach_calls
     - avoid_init_to_null
     # - avoid_js_rounded_ints # only useful when targeting JS runtime

examples/flutter_gallery/lib/demo/pesto_demo.dart

@@ -250,14 +250,14 @@ class _PestoLogoState extends State<PestoLogo> {
 
 // A card with the recipe's image, author, and title.
 class RecipeCard extends StatelessWidget {
-  final TextStyle titleStyle = const PestoStyle(fontSize: 24.0, fontWeight: FontWeight.w600);
-  final TextStyle authorStyle = const PestoStyle(fontWeight: FontWeight.w500, color: Colors.black54);
-
   const RecipeCard({ Key key, this.recipe, this.onTap }) : super(key: key);
 
   final Recipe recipe;
   final VoidCallback onTap;
 
+  TextStyle get titleStyle => const PestoStyle(fontSize: 24.0, fontWeight: FontWeight.w600);
+  TextStyle get authorStyle => const PestoStyle(fontWeight: FontWeight.w500, color: Colors.black54);
+
   @override
   Widget build(BuildContext context) {
     return new GestureDetector(

packages/flutter/lib/src/gestures/events.dart

@@ -176,7 +176,7 @@ abstract class PointerEvent {
   final double distance;
 
   /// The minimum value that a distance can return for this pointer (always 0.0).
-  final double distanceMin = 0.0;
+  double get distanceMin => 0.0;
 
   /// The maximum value that a distance can return for this pointer. If this
   /// input device cannot detect "hover touch" input events, then this will be