Start using protected annotation (#4288)

There are many more places we can use this annotation. This patch starts out
small to test the waters.

packages/flutter/lib/src/rendering/box.dart

@@ -7,6 +7,7 @@ import 'dart:ui' as ui show lerpDouble;
 
 import 'package:flutter/animation.dart';
 import 'package:flutter/gestures.dart';
+import 'package:meta/meta.dart';
 import 'package:vector_math/vector_math_64.dart';
 
 import 'debug.dart';
@@ -815,6 +816,7 @@ abstract class RenderBox extends RenderObject {
   ///
   /// Subclasses should override this function to supply the distances to their
   /// baselines.
+  @protected
   double computeDistanceToActualBaseline(TextBaseline baseline) {
     assert(_debugDoingBaseline);
     return null;
@@ -1035,6 +1037,7 @@ abstract class RenderBox extends RenderObject {
 
   /// Override this function if this render object can be hit even if its
   /// children were not hit.
+  @protected
   bool hitTestSelf(Point position) => false;
 
   /// Override this function to check whether any children are located at the
@@ -1043,6 +1046,7 @@ abstract class RenderBox extends RenderObject {
   /// Typically children should be hit tested in reverse paint order so that
   /// hit tests at locations where children overlap hit the child that is
   /// visually "on top" (i.e., paints later).
+  @protected
   bool hitTestChildren(HitTestResult result, { Point position }) => false;
 
   /// Multiply the transform from the parent's coordinate system to this box's
@@ -1149,6 +1153,7 @@ abstract class RenderBox extends RenderObject {
   /// In debug mode, paints a border around this render box.
   ///
   /// Called for every [RenderBox] when [debugPaintSizeEnabled] is true.
+  @protected
   void debugPaintSize(PaintingContext context, Offset offset) {
     assert(() {
       Paint paint = new Paint()
@@ -1163,6 +1168,7 @@ abstract class RenderBox extends RenderObject {
   /// In debug mode, paints a line for each baseline.
   ///
   /// Called for every [RenderBox] when [debugPaintBaselinesEnabled] is true.
+  @protected
   void debugPaintBaselines(PaintingContext context, Offset offset) {
     assert(() {
       Paint paint = new Paint()
@@ -1194,6 +1200,7 @@ abstract class RenderBox extends RenderObject {
   /// In debug mode, paints a rectangle if this render box has received more pointer downs than pointer up events.
   ///
   /// Called for every [RenderBox] when [debugPaintPointersEnabled] is true.
+  @protected
   void debugPaintPointers(PaintingContext context, Offset offset) {
     assert(() {
       if (_debugActivePointers > 0) {

packages/flutter/lib/src/rendering/object.dart

@@ -9,6 +9,7 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/gestures.dart';
 import 'package:flutter/painting.dart';
 import 'package:flutter/scheduler.dart';
+import 'package:meta/meta.dart';
 import 'package:mojo_services/mojo/gfx/composition/scene_token.mojom.dart' as mojom;
 import 'package:vector_math/vector_math_64.dart';
 
@@ -29,6 +30,7 @@ export 'package:flutter/foundation.dart' show FlutterError, InformationCollector
 /// to other children.
 class ParentData {
   /// Called when the RenderObject is removed from the tree.
+  @mustCallSuper
   void detach() { }
 
   @override
@@ -1438,6 +1440,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
   /// property isn't used when debugCanParentUseSize isn't set, then that
   /// subclass should override debugResetSize() to reapply the current values of
   /// debugCanParentUseSize to that state.
+  @protected
   void debugResetSize() { }
 
   /// Whether the constraints are the only input to the sizing algorithm (in
@@ -1459,6 +1462,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
   /// to compute their size.
   ///
   /// This function is called only if [sizedByParent] is true.
+  @protected
   void performResize();
 
   /// Do the work of computing the layout for this render object.
@@ -1480,6 +1484,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
   /// parentUsesSize ensures that this render object will undergo layout if the
   /// child undergoes layout. Otherwise, the child can changes its layout
   /// information without informing this render object.
+  @protected
   void performLayout();
 
   // We cache a closure to performLayout so that the callsite is monomorphic.
@@ -1489,6 +1494,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
 
   /// Allows this render object to mutate its child list during layout and
   /// calls callback.
+  @protected
   void invokeLayoutCallback(LayoutCallback callback) {
     assert(_debugMutationsLocked);
     assert(_debugDoingThisLayout);
@@ -1754,6 +1760,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
   Rect get paintBounds;
 
   /// Override this function to paint debugging information.
+  @protected
   void debugPaint(PaintingContext context, Offset offset) { }
 
   /// Paint this render object into the given context at the given offset.
@@ -2094,6 +2101,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
   /// Returns a list of strings describing the current node's fields, one field
   /// per string. Subclasses should override this to have their information
   /// included in toStringDeep().
+  @protected
   void debugFillDescription(List<String> description) {
     if (debugCreator != null)
       description.add('creator: $debugCreator');
@@ -2103,6 +2111,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
 
   /// Returns a string describing the current node's descendants. Each line of
   /// the subtree in the output should be indented by the prefix argument.
+  @protected
   String debugDescribeChildren(String prefix) => '';
 
 }