Implementing switch expressions in rendering/ (flutter#143812)

This pull request is part of the effort to solve issue flutter#136139.

The previous [`switch` expressions PR](flutter#143496) was comprised of many simple changes throughout `flutter/lib/src/`, but due to some more in-depth refactoring in `flutter/lib/src/rendering/`, I decided to submit the changes to this directory as a separate pull request.

There was really just one function that I changed significantly; I'll add a comment for explanation.

Author: nate-thegrate

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

@@ -93,32 +93,27 @@ class RenderDecoratedSliver extends RenderProxySliver {
 
   @override
   void paint(PaintingContext context, Offset offset) {
-    if (child != null && child!.geometry!.visible) {
-      final SliverPhysicalParentData childParentData = child!.parentData! as SliverPhysicalParentData;
-      final Size childSize;
-      final Offset scrollOffset;
-
-      // In the case where the child sliver has infinite scroll extent, the decoration
-      // should only extend down to the bottom cache extent.
-      final double cappedMainAxisExtent = child!.geometry!.scrollExtent.isInfinite
-        ? constraints.scrollOffset + child!.geometry!.cacheExtent + constraints.cacheOrigin
-        : child!.geometry!.scrollExtent;
-      switch (constraints.axis) {
-        case Axis.vertical:
-          childSize = Size(constraints.crossAxisExtent, cappedMainAxisExtent);
-          scrollOffset = Offset(0.0, -constraints.scrollOffset);
-        case Axis.horizontal:
-          childSize = Size(cappedMainAxisExtent, constraints.crossAxisExtent);
-          scrollOffset = Offset(-constraints.scrollOffset, 0.0);
-      }
-      final Offset childOffset = offset + childParentData.paintOffset;
-      if (position == DecorationPosition.background) {
-        _painter!.paint(context.canvas, childOffset + scrollOffset, configuration.copyWith(size: childSize));
-      }
-      context.paintChild(child!, childOffset);
-      if (position == DecorationPosition.foreground) {
-        _painter!.paint(context.canvas, childOffset + scrollOffset, configuration.copyWith(size: childSize));
-      }
+    if (child == null || !child!.geometry!.visible) {
+      return;
+    }
+    // In the case where the child sliver has infinite scroll extent, the decoration
+    // should only extend down to the bottom cache extent.
+    final double cappedMainAxisExtent = child!.geometry!.scrollExtent.isInfinite
+      ? constraints.scrollOffset + child!.geometry!.cacheExtent + constraints.cacheOrigin
+      : child!.geometry!.scrollExtent;
+    final (Size childSize, Offset scrollOffset) = switch (constraints.axis) {
+      Axis.horizontal => (Size(cappedMainAxisExtent, constraints.crossAxisExtent), Offset(-constraints.scrollOffset, 0.0)),
+      Axis.vertical   => (Size(constraints.crossAxisExtent, cappedMainAxisExtent), Offset(0.0, -constraints.scrollOffset)),
+    };
+    offset += (child!.parentData! as SliverPhysicalParentData).paintOffset;
+    void paintDecoration() => _painter!.paint(context.canvas, offset + scrollOffset, configuration.copyWith(size: childSize));
+    switch (position) {
+      case DecorationPosition.background:
+        paintDecoration();
+        context.paintChild(child!, offset);
+      case DecorationPosition.foreground:
+        context.paintChild(child!, offset);
+        paintDecoration();
     }
   }
 }

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

@@ -67,14 +67,11 @@ class TextSelectionPoint {
 
   @override
   String toString() {
-    switch (direction) {
-      case TextDirection.ltr:
-        return '$point-ltr';
-      case TextDirection.rtl:
-        return '$point-rtl';
-      case null:
-        return '$point';
-    }
+    return switch (direction) {
+      TextDirection.ltr => '$point-ltr',
+      TextDirection.rtl => '$point-rtl',
+      null => '$point',
+    };
   }
 
   @override
@@ -1686,32 +1683,26 @@ class RenderEditable extends RenderBox with RelayoutWhenSystemFontsChangeMixin,
   Axis get _viewportAxis => _isMultiline ? Axis.vertical : Axis.horizontal;
 
   Offset get _paintOffset {
-    switch (_viewportAxis) {
-      case Axis.horizontal:
-        return Offset(-offset.pixels, 0.0);
-      case Axis.vertical:
-        return Offset(0.0, -offset.pixels);
-    }
+    return switch (_viewportAxis) {
+      Axis.horizontal => Offset(-offset.pixels, 0.0),
+      Axis.vertical   => Offset(0.0, -offset.pixels),
+    };
   }
 
   double get _viewportExtent {
     assert(hasSize);
-    switch (_viewportAxis) {
-      case Axis.horizontal:
-        return size.width;
-      case Axis.vertical:
-        return size.height;
-    }
+    return switch (_viewportAxis) {
+      Axis.horizontal => size.width,
+      Axis.vertical   => size.height,
+    };
   }
 
   double _getMaxScrollExtent(Size contentSize) {
     assert(hasSize);
-    switch (_viewportAxis) {
-      case Axis.horizontal:
-        return math.max(0.0, contentSize.width - size.width);
-      case Axis.vertical:
-        return math.max(0.0, contentSize.height - size.height);
-    }
+    return switch (_viewportAxis) {
+      Axis.horizontal => math.max(0.0, contentSize.width - size.width),
+      Axis.vertical   => math.max(0.0, contentSize.height - size.height),
+    };
   }
 
   // We need to check the paint offset here because during animation, the start of