Test what context the overlay entries build in (#6337)

Fixes #6245

packages/flutter/test/widget/overlay_test.dart

+// Copyright 2016 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/widgets.dart';
+
+void main() {
+  testWidgets('OverflowEntries context contains Overlay',
+      (WidgetTester tester) async {
+    Key overlayKey = new UniqueKey();
+    bool didBuild = false;
+    await tester.pumpWidget(new Overlay(
+      key: overlayKey,
+      initialEntries: <OverlayEntry>[
+        new OverlayEntry(
+          builder: (BuildContext context) {
+            didBuild = true;
+            Overlay overlay = context.ancestorWidgetOfExactType(Overlay);
+            expect(overlay, isNotNull);
+            expect(overlay.key, equals(overlayKey));
+            return new Container();
+          },
+        )
+      ],
+    ));
+    expect(didBuild, isTrue);
+  });
+}