Handle empty sizes in AppBar's toolbar layout (#6894)

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

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:math' as math;
+
 import 'package:flutter/services.dart';
 import 'package:flutter/widgets.dart';
 
@@ -58,7 +60,7 @@ class _ToolbarLayout extends MultiChildLayoutDelegate {
     }
 
     if (hasChild(_ToolbarSlot.title)) {
-      final double maxWidth = size.width - kTitleLeft - actionsWidth;
+      final double maxWidth = math.max(size.width - kTitleLeft - actionsWidth, 0.0);
       final BoxConstraints constraints = new BoxConstraints.loose(size).copyWith(maxWidth: maxWidth);
       final Size titleSize = layoutChild(_ToolbarSlot.title, constraints);
       final double titleY = (size.height - titleSize.height) / 2.0;

packages/flutter/test/material/app_bar_test.dart

@@ -179,4 +179,22 @@ void main() {
     expect(tester.getSize(title).width, equals(620.0));
   });
 
+  testWidgets('AppBar render at zero size', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new Center(
+        child: new Container(
+          height: 0.0,
+          width: 0.0,
+          child: new Scaffold(
+            appBar: new AppBar(
+              title: new Text('X')
+            )
+          )
+        )
+      )
+    );
+
+    Finder title = find.text('X');
+    expect(tester.getSize(title).isEmpty, isTrue);
+  });
 }