Explicit import for Platform and kIsWeb (#1898)

Following the ideas in flutter/website#7798 I have changed the imports
needed for Platform and kIsWeb to be explicitly named.

## Pre-launch Checklist

- [ ] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [ ] I signed the [CLA].
- [ ] I read the [Contributors Guide].
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [ ] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel
channel on [Discord].

<!-- Links -->
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[CLA]: https://cla.developers.google.com/
[Discord]: https://github.com/flutter/flutter/wiki/Chat
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md

animations/lib/main.dart

@@ -2,9 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'dart:io';
+import 'dart:io' show Platform;
 
-import 'package:flutter/foundation.dart';
+import 'package:flutter/foundation.dart' show kIsWeb;
 import 'package:flutter/material.dart';
 import 'package:go_router/go_router.dart';
 import 'package:window_size/window_size.dart';

code_sharing/client/lib/main.dart

@@ -1,5 +1,5 @@
 import 'dart:convert';
-import 'dart:io';
+import 'dart:io' show Platform;
 import 'package:http/http.dart' as http;
 import 'package:shared/shared.dart';
 import 'package:flutter/material.dart';

form_app/lib/main.dart

@@ -2,9 +2,9 @@
 // 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 'dart:io';
+import 'dart:io' show Platform;
 
-import 'package:flutter/foundation.dart';
+import 'package:flutter/foundation.dart' show kIsWeb;
 import 'package:flutter/material.dart';
 import 'package:go_router/go_router.dart';
 import 'package:window_size/window_size.dart';

infinite_list/lib/main.dart

@@ -2,9 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'dart:io';
+import 'dart:io' show Platform;
 
-import 'package:flutter/foundation.dart';
+import 'package:flutter/foundation.dart' show kIsWeb;
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
 import 'package:window_size/window_size.dart';

isolate_example/lib/main.dart

@@ -12,9 +12,9 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-import 'dart:io';
+import 'dart:io' show Platform;
 
-import 'package:flutter/foundation.dart';
+import 'package:flutter/foundation.dart' show kIsWeb;
 import 'package:flutter/material.dart';
 import 'package:window_size/window_size.dart';
 

jsonexample/lib/main.dart

@@ -2,9 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'dart:io';
+import 'dart:io' show Platform;
 
-import 'package:flutter/foundation.dart';
+import 'package:flutter/foundation.dart' show kIsWeb;
 import 'package:flutter/material.dart';
 import 'package:jsonexample/tab_pages.dart';
 import 'package:window_size/window_size.dart';

navigation_and_routing/lib/main.dart

@@ -2,9 +2,9 @@
 // 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 'dart:io';
+import 'dart:io' show Platform;
 
-import 'package:flutter/foundation.dart';
+import 'package:flutter/foundation.dart' show kIsWeb;
 import 'package:flutter/material.dart';
 import 'package:url_strategy/url_strategy.dart';
 import 'package:window_size/window_size.dart';

provider_counter/lib/main.dart

@@ -2,9 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'dart:io';
+import 'dart:io' show Platform;
 
-import 'package:flutter/foundation.dart';
+import 'package:flutter/foundation.dart' show kIsWeb;
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
 import 'package:window_size/window_size.dart';

provider_shopper/lib/main.dart

@@ -2,9 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'dart:io';
+import 'dart:io' show Platform;
 
-import 'package:flutter/foundation.dart';
+import 'package:flutter/foundation.dart' show kIsWeb;
 import 'package:flutter/material.dart';
 import 'package:go_router/go_router.dart';
 import 'package:provider/provider.dart';

simplistic_calculator/lib/main.dart

@@ -2,10 +2,10 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'dart:io';
+import 'dart:io' show Platform;
 
 import 'package:auto_size_text/auto_size_text.dart';
-import 'package:flutter/foundation.dart';
+import 'package:flutter/foundation.dart' show kIsWeb;
 import 'package:flutter/material.dart';
 import 'package:flutter_layout_grid/flutter_layout_grid.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';

veggieseasons/lib/main.dart

@@ -2,10 +2,10 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'dart:io';
+import 'dart:io' show Platform;
 
 import 'package:flutter/cupertino.dart';
-import 'package:flutter/foundation.dart';
+import 'package:flutter/foundation.dart' show kIsWeb;
 import 'package:flutter/services.dart' show DeviceOrientation, SystemChrome;
 import 'package:go_router/go_router.dart';
 import 'package:provider/provider.dart';