Add a flag to allow the default Client to be tree shaken away. (#868)

pkgs/http/CHANGELOG.md

@@ -1,6 +1,8 @@
 ## 0.13.6-dev
 
 * `BrowserClient` throws an exception if `send` is called after `close`.
+* If `no_default_http_client=true` is set in the environment then disk usage
+  is reduced in some circumstances.
 * Require Dart 2.19
 
 ## 0.13.5

pkgs/http/lib/src/browser_client.dart

@@ -15,7 +15,13 @@ import 'streamed_response.dart';
 /// Create a [BrowserClient].
 ///
 /// Used from conditional imports, matches the definition in `client_stub.dart`.
-BaseClient createClient() => BrowserClient();
+BaseClient createClient() {
+  if (const bool.fromEnvironment('no_default_http_client')) {
+    throw StateError('no_default_http_client was defined but runWithClient '
+        'was not used to configure a Client implementation.');
+  }
+  return BrowserClient();
+}
 
 /// A `dart:html`-based HTTP client that runs in the browser and is backed by
 /// XMLHttpRequests.

pkgs/http/lib/src/client.dart

@@ -210,6 +210,17 @@ Client? get zoneClient {
 ///  runWithClient(() => runApp(const MyApp()), clientFactory);
 /// }
 /// ```
+///
+/// If [runWithClient] is used and the environment defines
+/// `no_default_http_client=true` then generated binaries may be smaller e.g.
+/// ```shell
+/// $ flutter build appbundle --dart-define=no_default_http_client=true ...
+/// $ dart compile exe --define=no_default_http_client=true ...
+/// ```
+///
+/// If `no_default_http_client=true` is set then any call to the [Client]
+/// factory (i.e. `Client()`) outside of the [Zone] created by [runWithClient]
+/// will throw [StateError].
 R runWithClient<R>(R Function() body, Client Function() clientFactory,
         {ZoneSpecification? zoneSpecification}) =>
     runZoned(body,

pkgs/http/lib/src/io_client.dart

@@ -12,7 +12,13 @@ import 'io_streamed_response.dart';
 /// Create an [IOClient].
 ///
 /// Used from conditional imports, matches the definition in `client_stub.dart`.
-BaseClient createClient() => IOClient();
+BaseClient createClient() {
+  if (const bool.fromEnvironment('no_default_http_client')) {
+    throw StateError('no_default_http_client was defined but runWithClient '
+        'was not used to configure a Client implementation.');
+  }
+  return IOClient();
+}
 
 /// Exception thrown when the underlying [HttpClient] throws a
 /// [SocketException].

pkgs/http/mono_pkg.yaml

@@ -15,3 +15,6 @@ stages:
   - test: --platform chrome
     os:
     - linux
+  - command: dart run --define=no_default_http_client=true test/no_default_http_client_test.dart
+    os:
+    - linux

pkgs/http/test/no_default_http_client_test.dart

@@ -0,0 +1,18 @@
+// Copyright (c) 2023, the Dart project authors.  Please see the AUTHORS file
+// for details. 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:http/http.dart' as http;
+import 'package:test/test.dart';
+
+/// Tests that no [http.Client] is provided by default when run with
+/// `--define=no_default_http_client=true`.
+void main() {
+  test('Client()', () {
+    if (const bool.fromEnvironment('no_default_http_client')) {
+      expect(http.Client.new, throwsA(isA<StateError>()));
+    } else {
+      expect(http.Client(), isA<http.Client>());
+    }
+  });
+}