mark widget == and hashCode as nonVirtual (#46900)

packages/flutter/lib/foundation.dart

@@ -12,6 +12,7 @@ library foundation;
 export 'package:meta/meta.dart' show
   immutable,
   mustCallSuper,
+  nonVirtual,
   optionalTypeArgs,
   protected,
   required,

packages/flutter/lib/src/widgets/framework.dart

@@ -451,6 +451,13 @@ abstract class Widget extends DiagnosticableTree {
     properties.defaultDiagnosticsTreeStyle = DiagnosticsTreeStyle.dense;
   }
 
+  @override
+  @nonVirtual
+  bool operator ==(dynamic other) => super==other;
+
+  @override
+  @nonVirtual
+  int get hashCode => super.hashCode;
 
   /// Whether the `newWidget` can be used to update an [Element] that currently
   /// has the `oldWidget` as its configuration.