Make FittedBox not throw when child has zero size. (#150430)

fix https://github.com/flutter/flutter/issues/135082 , fix https://github.com/flutter/flutter/issues/139539 , fix https://github.com/flutter/flutter/issues/142910

Before, `FittedBox` would throw when child size is zero, unless the constraint is tight and fit is not `BoxFit.scaleDown`.

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

@@ -322,10 +322,12 @@ class BoxConstraints extends Constraints {
       return result;
     }
 
+    if (size.isEmpty) {
+      return constrain(size);
+    }
+
     double width = size.width;
     double height = size.height;
-    assert(width > 0.0);
-    assert(height > 0.0);
     final double aspectRatio = width / height;
 
     if (width > maxWidth) {

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

@@ -2711,24 +2711,6 @@ class RenderFittedBox extends RenderProxyBox {
     if (child != null) {
       final Size childSize = child!.getDryLayout(const BoxConstraints());
 
-      // During [RenderObject.debugCheckingIntrinsics] a child that doesn't
-      // support dry layout may provide us with an invalid size that triggers
-      // assertions if we try to work with it. Instead of throwing, we bail
-      // out early in that case.
-      bool invalidChildSize = false;
-      assert(() {
-        if (RenderObject.debugCheckingIntrinsics && childSize.width * childSize.height == 0.0) {
-          invalidChildSize = true;
-        }
-        return true;
-      }());
-      if (invalidChildSize) {
-        assert(debugCannotComputeDryLayout(
-          reason: 'Child provided invalid size of $childSize.',
-        ));
-        return Size.zero;
-      }
-
       switch (fit) {
         case BoxFit.scaleDown:
           final BoxConstraints sizeConstraints = constraints.loosen();

packages/flutter/test/rendering/box_constraints_test.dart

@@ -182,4 +182,17 @@ void main() {
     expect(constraints, const BoxConstraints(minWidth: 1, maxWidth: 2, minHeight: 3, maxHeight: 4));
   });
 
+  test('BoxConstraints.constrainSizeAndAttemptToPreserveAspectRatio can handle empty size', () {
+    const BoxConstraints constraints = BoxConstraints(
+      minWidth: 10.0,
+      maxWidth: 20.0,
+      minHeight: 10.0,
+      maxHeight: 20.0,
+    );
+    const Size unconstrainedSize = Size(15.0, 0.0);
+    final Size constrainedSize = constraints.constrainSizeAndAttemptToPreserveAspectRatio(
+      unconstrainedSize,
+    );
+    expect(constrainedSize, const Size(15.0, 10.0));
+  });
 }

packages/flutter/test/widgets/fitted_box_test.dart

@@ -605,6 +605,38 @@ void main() {
     await tester.tap(find.byType(FittedBox), warnIfMissed: false);
     expect(tester.takeException(), isNull);
   });
+
+  // Regression test for https://github.com/flutter/flutter/issues/135082
+  testWidgets('FittedBox with zero size child does not throw', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const Center(
+        child: SizedBox(
+          height: 200.0,
+          width: 200.0,
+          child: FittedBox(
+            fit: BoxFit.scaleDown,
+            child: SizedBox.shrink(),
+          ),
+        ),
+      ),
+    );
+    expect(tester.takeException(), isNull);
+
+    await tester.pumpWidget(
+      Center(
+        child: ConstrainedBox(
+          constraints: const BoxConstraints(
+            maxWidth: 200.0,
+            maxHeight: 200.0,
+          ),
+          child: const FittedBox(
+            child: SizedBox.shrink(),
+          ),
+        ),
+      ),
+    );
+    expect(tester.takeException(), isNull);
+  });
 }
 
 List<Type> getLayers() {