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.

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[Flutter Style Guide]:
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[Contributors Guide]:
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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';