Deprecate redundant itemExtent in RenderSliverFixedExtentBoxAdaptor methods (#143412)

Multiple methods in `RenderSliverFixedExtentBoxAdaptor` pass a `double itemExtent` for computing things like what children will be laid out, what the max scroll offset will be, and how the children will be laid out.

Since `RenderSliverFixedExtentBoxAdaptor` was further subclassed to support a `itemExtentBuider` in `RenderSliverVariedExtentList`, these itemExtent parameters became useless when using that RenderObject. Reading through `RenderSliverFixedExtentBoxAdaptor.performLayout`, the remaining artifacts of passing around itemExtent make it hard to follow when it is irrelevant. 

`RenderSliverFixedExtentBoxAdaptor.itemExtent` is available from these methods, so it does not need to pass it. It is redundant API.

Plus, if a bogus itemExtent is passed for some reason, errors will ensue and the layout will be incorrect. 💣 💥 

Deprecating so we can remove these for a cleaner API. Unfortunately this is not supported by dart fix, but the fact that these methods are protected means usage outside of the framework is likely minimal.

packages/flutter/test/rendering/sliver_fixed_extent_layout_test.dart

@@ -148,10 +148,208 @@ void main() {
     expect(parent.paintsChild(children[1]), false);
     expect(parent.paintsChild(children[2]), true);
   });
+
+  test('RenderSliverFillViewport correctly references itemExtent, non-zero offset', () {
+    final List<RenderBox> children = <RenderBox>[
+      RenderSizedBox(const Size(400.0, 100.0)),
+      RenderSizedBox(const Size(400.0, 100.0)),
+      RenderSizedBox(const Size(400.0, 100.0)),
+    ];
+    final TestRenderSliverBoxChildManager childManager = TestRenderSliverBoxChildManager(
+      children: children,
+    );
+    final RenderSliverFillViewport sliver = childManager.createRenderSliverFillViewport();
+    final RenderViewport root = RenderViewport(
+      crossAxisDirection: AxisDirection.right,
+      offset: ViewportOffset.fixed(1200.0),
+      cacheExtent: 100,
+      children: <RenderSliver>[ sliver ],
+    );
+    layout(root);
+    // These are a bogus itemExtents, and motivate the deprecation. The sliver
+    // knows its itemExtent, and should use its configured extent rather than
+    // whatever is provided through these methods.
+    // Also, the API is a bit redundant, so we clean!
+    // In this case, the true item extent is 600 to fill the viewport.
+    expect(
+      sliver.constraints.scrollOffset,
+      1200.0
+    );
+    expect(sliver.itemExtent, 600.0);
+    final double layoutOffset = sliver.indexToLayoutOffset(
+      150.0, // itemExtent
+      10,
+    );
+    expect(layoutOffset, 6000.0);
+    final int minIndex = sliver.getMinChildIndexForScrollOffset(
+      sliver.constraints.scrollOffset,
+      150.0, // itemExtent
+    );
+    expect(minIndex, 2);
+    final int maxIndex = sliver.getMaxChildIndexForScrollOffset(
+      sliver.constraints.scrollOffset,
+      150, // itemExtent
+    );
+    expect(maxIndex, 1);
+    final double maxScrollOffset = sliver.computeMaxScrollOffset(
+      sliver.constraints,
+      150, // itemExtent
+    );
+    expect(maxScrollOffset, 1800.0);
+  });
+
+  test('RenderSliverFillViewport correctly references itemExtent, zero offset', () {
+    final List<RenderBox> children = <RenderBox>[
+      RenderSizedBox(const Size(400.0, 100.0)),
+      RenderSizedBox(const Size(400.0, 100.0)),
+      RenderSizedBox(const Size(400.0, 100.0)),
+    ];
+    final TestRenderSliverBoxChildManager childManager = TestRenderSliverBoxChildManager(
+      children: children,
+    );
+    final RenderSliverFillViewport sliver = childManager.createRenderSliverFillViewport();
+    final RenderViewport root = RenderViewport(
+      crossAxisDirection: AxisDirection.right,
+      offset: ViewportOffset.zero(),
+      cacheExtent: 100,
+      children: <RenderSliver>[ sliver ],
+    );
+    layout(root);
+    // These are a bogus itemExtents, and motivate the deprecation. The sliver
+    // knows its itemExtent, and should use its configured extent rather than
+    // whatever is provided through these methods.
+    // Also, the API is a bit redundant, so we clean!
+    // In this case, the true item extent is 600 to fill the viewport.
+    expect(
+      sliver.constraints.scrollOffset,
+      0.0
+    );
+    expect(sliver.itemExtent, 600.0);
+    final double layoutOffset = sliver.indexToLayoutOffset(
+      150.0, // itemExtent
+      10,
+    );
+    expect(layoutOffset, 6000.0);
+    final int minIndex = sliver.getMinChildIndexForScrollOffset(
+      sliver.constraints.scrollOffset,
+      150.0, // itemExtent
+    );
+    expect(minIndex, 0);
+    final int maxIndex = sliver.getMaxChildIndexForScrollOffset(
+      sliver.constraints.scrollOffset,
+      150, // itemExtent
+    );
+    expect(maxIndex, 0);
+    final double maxScrollOffset = sliver.computeMaxScrollOffset(
+      sliver.constraints,
+      150, // itemExtent
+    );
+    expect(maxScrollOffset, 1800.0);
+  });
+
+  test('RenderSliverFixedExtentList correctly references itemExtent, non-zero offset', () {
+    final List<RenderBox> children = <RenderBox>[
+      RenderSizedBox(const Size(400.0, 100.0)),
+      RenderSizedBox(const Size(400.0, 100.0)),
+      RenderSizedBox(const Size(400.0, 100.0)),
+    ];
+    final TestRenderSliverBoxChildManager childManager = TestRenderSliverBoxChildManager(
+      children: children,
+    );
+    final RenderSliverFixedExtentList sliver = childManager.createRenderSliverFixedExtentList(30.0);
+    final RenderViewport root = RenderViewport(
+      crossAxisDirection: AxisDirection.right,
+      offset: ViewportOffset.fixed(45.0),
+      cacheExtent: 100,
+      children: <RenderSliver>[ sliver ],
+    );
+    layout(root);
+    // These are a bogus itemExtents, and motivate the deprecation. The sliver
+    // knows its itemExtent, and should use its configured extent rather than
+    // whatever is provided through these methods.
+    // Also, the API is a bit redundant, so we clean!
+    // In this case, the true item extent is 30.0.
+    expect(
+      sliver.constraints.scrollOffset,
+      45.0
+    );
+    expect(sliver.constraints.viewportMainAxisExtent, 600.0);
+    expect(sliver.itemExtent, 30.0);
+    final double layoutOffset = sliver.indexToLayoutOffset(
+      150.0, // itemExtent
+      10,
+    );
+    expect(layoutOffset, 300.0);
+    final int minIndex = sliver.getMinChildIndexForScrollOffset(
+      sliver.constraints.scrollOffset,
+      150.0, // itemExtent
+    );
+    expect(minIndex, 1);
+    final int maxIndex = sliver.getMaxChildIndexForScrollOffset(
+      sliver.constraints.scrollOffset,
+      150, // itemExtent
+    );
+    expect(maxIndex, 1);
+    final double maxScrollOffset = sliver.computeMaxScrollOffset(
+      sliver.constraints,
+      150, // itemExtent
+    );
+    expect(maxScrollOffset, 90.0);
+  });
+
+  test('RenderSliverFixedExtentList correctly references itemExtent, zero offset', () {
+    final List<RenderBox> children = <RenderBox>[
+      RenderSizedBox(const Size(400.0, 100.0)),
+      RenderSizedBox(const Size(400.0, 100.0)),
+      RenderSizedBox(const Size(400.0, 100.0)),
+    ];
+    final TestRenderSliverBoxChildManager childManager = TestRenderSliverBoxChildManager(
+      children: children,
+    );
+    final RenderSliverFixedExtentList sliver = childManager.createRenderSliverFixedExtentList(30.0);
+    final RenderViewport root = RenderViewport(
+      crossAxisDirection: AxisDirection.right,
+      offset: ViewportOffset.zero(),
+      cacheExtent: 100,
+      children: <RenderSliver>[ sliver ],
+    );
+    layout(root);
+    // These are a bogus itemExtents, and motivate the deprecation. The sliver
+    // knows its itemExtent, and should use its configured extent rather than
+    // whatever is provided through these methods.
+    // Also, the API is a bit redundant, so we clean!
+    // In this case, the true item extent is 30.0.
+    expect(
+      sliver.constraints.scrollOffset,
+      0.0
+    );
+    expect(sliver.constraints.viewportMainAxisExtent, 600.0);
+    expect(sliver.itemExtent, 30.0);
+    final double layoutOffset = sliver.indexToLayoutOffset(
+      150.0, // itemExtent
+      10,
+    );
+    expect(layoutOffset, 300.0);
+    final int minIndex = sliver.getMinChildIndexForScrollOffset(
+      sliver.constraints.scrollOffset,
+      150.0, // itemExtent
+    );
+    expect(minIndex, 0);
+    final int maxIndex = sliver.getMaxChildIndexForScrollOffset(
+      sliver.constraints.scrollOffset,
+      150, // itemExtent
+    );
+    expect(maxIndex, 0);
+    final double maxScrollOffset = sliver.computeMaxScrollOffset(
+      sliver.constraints,
+      150, // itemExtent
+    );
+    expect(maxScrollOffset, 90.0);
+  });
 }
 
 int testGetMaxChildIndexForScrollOffset(double scrollOffset, double itemExtent) {
-  final TestRenderSliverFixedExtentBoxAdaptor renderSliver = TestRenderSliverFixedExtentBoxAdaptor();
+  final TestRenderSliverFixedExtentBoxAdaptor renderSliver = TestRenderSliverFixedExtentBoxAdaptor(itemExtent: itemExtent);
   return renderSliver.getMaxChildIndexForScrollOffset(scrollOffset, itemExtent);
 }
 
@@ -171,6 +369,15 @@ class TestRenderSliverBoxChildManager extends RenderSliverBoxChildManager {
     return _renderObject! as RenderSliverFillViewport;
   }
 
+  RenderSliverFixedExtentList createRenderSliverFixedExtentList(double itemExtent) {
+    assert(_renderObject == null);
+    _renderObject = RenderSliverFixedExtentList(
+      childManager: this,
+      itemExtent: itemExtent,
+    );
+    return _renderObject! as RenderSliverFixedExtentList;
+  }
+
   int? _currentlyUpdatingChildIndex;
 
   @override
@@ -218,8 +425,12 @@ class TestRenderSliverBoxChildManager extends RenderSliverBoxChildManager {
 }
 
 class TestRenderSliverFixedExtentBoxAdaptor extends RenderSliverFixedExtentBoxAdaptor {
-  TestRenderSliverFixedExtentBoxAdaptor()
-    :super(childManager: TestRenderSliverBoxChildManager(children: <RenderBox>[]));
+  TestRenderSliverFixedExtentBoxAdaptor({
+    required double itemExtent
+  }) : _itemExtent = itemExtent,
+       super(childManager: TestRenderSliverBoxChildManager(children: <RenderBox>[]));
+
+  final double _itemExtent;
 
   @override
   int getMaxChildIndexForScrollOffset(double scrollOffset, double itemExtent) {
@@ -227,5 +438,5 @@ class TestRenderSliverFixedExtentBoxAdaptor extends RenderSliverFixedExtentBoxAd
   }
 
   @override
-  double get itemExtent => throw UnimplementedError();
+  double get itemExtent => _itemExtent;
 }