Add:创建 flutter go web 版

packages/flutter_web/test/widgets/sliver_constraints_test.dart

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+// Copyright 2019 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_web/material.dart';
+import 'package:flutter_web/rendering.dart';
+import 'package:flutter_web/widgets.dart';
+import 'package:flutter_web_test/flutter_web_test.dart';
+
+void main() {
+  testWidgets(
+      'precedingScrollExtent is reported as infinity for Sliver of unknown size',
+      (WidgetTester tester) async {
+    await tester.pumpWidget(
+      MaterialApp(
+        home: CustomScrollView(
+          slivers: <Widget>[
+            const SliverToBoxAdapter(
+                child: SizedBox(width: double.infinity, height: 150.0)),
+            const SliverToBoxAdapter(
+                child: SizedBox(width: double.infinity, height: 150.0)),
+            SliverList(
+              delegate:
+                  SliverChildBuilderDelegate((BuildContext context, int index) {
+                if (index < 100) {
+                  return const SizedBox(width: double.infinity, height: 150.0);
+                } else {
+                  return null;
+                }
+              }),
+            ),
+            const SliverToBoxAdapter(
+              key: Key('final_sliver'),
+              child: SizedBox(width: double.infinity, height: 150.0),
+            ),
+          ],
+        ),
+      ),
+    );
+
+    // The last Sliver comes after a SliverList that has many more items than
+    // can fit in the viewport, and the SliverList doesn't report a child count,
+    // so the SliverList leads to an infinite precedingScrollExtent.
+    final RenderViewport renderViewport =
+        tester.renderObject(find.byType(Viewport));
+    final RenderSliver lastRenderSliver = renderViewport.lastChild;
+    expect(lastRenderSliver.constraints.precedingScrollExtent, double.infinity);
+  });
+}