Fix overlap check in bottom app bar's custom clipper (#14813)

packages/flutter/lib/src/material/bottom_app_bar.dart

@@ -117,7 +117,7 @@ class _BottomAppBarClipper extends CustomClipper<Path> {
     final Rect button = geometry.value.floatingActionButtonArea
       .translate(0.0, geometry.value.bottomNavigationBarTop * -1.0);
 
-    if (appBar.overlaps(button)) {
+    if (!appBar.overlaps(button)) {
       return new Path()..addRect(appBar);
     }
 

packages/flutter/test/material/bottom_app_bar_test.dart

@@ -0,0 +1,101 @@
+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
+
+void main() {
+  testWidgets('no overlap with floating action button', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new MaterialApp(
+        home: const Scaffold(
+          floatingActionButton: const FloatingActionButton(
+            onPressed: null,
+          ),
+          bottomNavigationBar: const ShapeListener(const BottomAppBar()),
+        ),
+      ),
+    );
+
+    final ShapeListenerState shapeListenerState = tester.state(find.byType(ShapeListener));
+    final RenderBox renderBox = tester.renderObject(find.byType(BottomAppBar));
+    final Path expectedPath = new Path()
+      ..addRect(Offset.zero & renderBox.size);
+
+    final Path actualPath = shapeListenerState.cache.value;
+    expect(
+      actualPath,
+      coversSameAreaAs(
+        expectedPath,
+        areaToCompare: (Offset.zero & renderBox.size).inflate(5.0),
+      )
+    );
+  });
+}
+
+// The bottom app bar clip path computation is only available at paint time.
+// In order to examine the notch path we implement this caching painter which
+// at paint time looks for for a descendant PhysicalShape and caches the
+// clip path it is using.
+class ClipCachePainter extends CustomPainter {
+  ClipCachePainter(this.context);
+
+  Path value;
+  BuildContext context;
+
+  @override
+  void paint(Canvas canvas, Size size) {
+    final RenderPhysicalShape physicalShape = findPhysicalShapeChild(context);
+    value = physicalShape.clipper.getClip(size);
+  }
+
+  RenderPhysicalShape findPhysicalShapeChild(BuildContext context) {
+    RenderPhysicalShape result;
+    context.visitChildElements((Element e) {
+      final RenderObject renderObject = e.findRenderObject();
+      if (renderObject.runtimeType == RenderPhysicalShape) {
+        assert(result == null);
+        result = renderObject;
+      } else {
+        result = findPhysicalShapeChild(e);
+      }
+    });
+    return result;
+  }
+
+  @override
+  bool shouldRepaint(ClipCachePainter oldDelegate) {
+    return true;
+  }
+}
+
+class ShapeListener extends StatefulWidget {
+  const ShapeListener(this.child);
+
+  final Widget child;
+
+  @override
+  State createState() => new ShapeListenerState();
+
+}
+
+class ShapeListenerState extends State<ShapeListener> {
+  @override
+  Widget build(BuildContext context) {
+    return new CustomPaint(
+      child: widget.child,
+      painter: cache
+    );
+  }
+
+  ClipCachePainter cache;
+
+  @override
+  void didChangeDependencies() {
+    super.didChangeDependencies();
+    cache = new ClipCachePainter(context);
+  }
+
+}