Be clearer about when and why we override HttpClient in tests (#49844)

dev/bots/test.dart

@@ -466,6 +466,30 @@ Future<void> _runFrameworkTests() async {
         'FLUTTER_EXPERIMENTAL_BUILD': 'true',
       },
     );
+    const String httpClientWarning =
+      'Warning: At least one test in this suite creates an HttpClient. When\n'
+      'running a test suite that uses TestWidgetsFlutterBinding, all HTTP\n'
+      'requests will return status code 400, and no network request will\n'
+      'actually be made. Any test expecting an real network connection and\n'
+      'status code will fail.\n'
+      'To test code that needs an HttpClient, provide your own HttpClient\n'
+      'implementation to the code under test, so that your test can\n'
+      'consistently provide a testable response to the code under test.';
+    await _runFlutterTest(
+      path.join(flutterRoot, 'packages', 'flutter_test'),
+      script: path.join('test', 'bindings_test_failure.dart'),
+      expectFailure: true,
+      printOutput: false,
+      outputChecker: (CapturedOutput output) {
+        final Iterable<Match> matches = httpClientWarning.allMatches(output.stdout);
+        if (matches == null || matches.isEmpty || matches.length > 1) {
+          return 'Failed to print warning about HttpClientUsage, or printed it too many times.\n'
+                 'stdout:\n${output.stdout}';
+        }
+        return null;
+      },
+      tableData: bigqueryApi?.tabledata,
+    );
   }
 
   await selectSubshard(<String, ShardRunner>{

packages/flutter_test/lib/src/_binding_io.dart

@@ -11,6 +11,9 @@ import 'package:flutter/services.dart';
 import 'package:flutter/widgets.dart';
 import 'package:meta/meta.dart';
 import 'package:path/path.dart' as path;
+// ignore: deprecated_member_use
+import 'package:test_api/test_api.dart' as test_package;
+
 
 import 'binding.dart';
 
@@ -73,8 +76,21 @@ void mockFlutterAssets() {
 /// If another [HttpClient] is provided using [HttpOverrides.runZoned], that will
 /// take precedence over this provider.
 class _MockHttpOverrides extends HttpOverrides {
+  bool warningPrinted = false;
   @override
   HttpClient createHttpClient(SecurityContext _) {
+    if (!warningPrinted) {
+      test_package.printOnFailure(
+        'Warning: At least one test in this suite creates an HttpClient. When\n'
+        'running a test suite that uses TestWidgetsFlutterBinding, all HTTP\n'
+        'requests will return status code 400, and no network request will\n'
+        'actually be made. Any test expecting an real network connection and\n'
+        'status code will fail.\n'
+        'To test code that needs an HttpClient, provide your own HttpClient\n'
+        'implementation to the code under test, so that your test can\n'
+        'consistently provide a testable response to the code under test.');
+      warningPrinted = true;
+    }
     return _MockHttpClient();
   }
 }

packages/flutter_test/lib/src/binding.dart

@@ -147,6 +147,13 @@ class TestDefaultBinaryMessenger extends BinaryMessenger {
 ///
 /// When using these bindings, certain features are disabled. For
 /// example, [timeDilation] is reset to 1.0 on initialization.
+///
+/// In non-browser tests, the binding overrides `HttpClient` creation with a
+/// fake client that always returns a status code of 400. This is to prevent
+/// tests from making network calls, which could introduce flakiness. A test
+/// that actually needs to make a network call should provide its own
+/// `HttpClient` to the code making the call, so that it can appropriately mock
+/// or fake responses.
 abstract class TestWidgetsFlutterBinding extends BindingBase
   with ServicesBinding,
        SchedulerBinding,

packages/flutter_test/test/bindings_test_failure.dart

@@ -0,0 +1,26 @@
+// Copyright 2014 The Flutter 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 'dart:io';
+
+import 'package:flutter_test/flutter_test.dart';
+
+// This test checks that we output something on test failure where the test
+// creates an HttpClient.
+// It should not be run as part of a normal test suite, as it is expected to
+// fail. See dev/bots/test.dart, which runs this test, checks that it fails,
+// and prints the expected warning about HttpClient usage.
+// We don't run this for browser tests, since we don't override the behavior
+// in browser tests.
+
+void main() {
+  test('Http Warning Message', () {
+    TestWidgetsFlutterBinding.ensureInitialized();
+    HttpClient();
+    // Make sure we only add the message once.
+    HttpClient();
+    HttpClient();
+    fail('Intentional');
+  });
+}