Remove package:platform source (#6898)

The source code for `package:platform` has been moved to https://github.com/dart-lang/platform/, see dart-archive/platform#46

This PR removes the source from this repo, and leave a small cookie trail README in place.

README.md

@@ -65,7 +65,6 @@ These are the packages hosted in this repository:
 | [palette\_generator](./packages/palette_generator/) | [![pub package](https://img.shields.io/pub/v/palette_generator.svg)](https://pub.dev/packages/palette_generator) | [![pub points](https://img.shields.io/pub/points/palette_generator)](https://pub.dev/packages/palette_generator/score) | [![popularity](https://img.shields.io/pub/popularity/palette_generator)](https://pub.dev/packages/palette_generator/score) | [![GitHub issues by-label](https://img.shields.io/github/issues/flutter/flutter/p%3A%20palette_generator?label=)](https://github.com/flutter/flutter/labels/p%3A%20palette_generator) | [![GitHub pull requests by-label](https://img.shields.io/github/issues-pr/flutter/packages/p%3A%20palette_generator?label=)](https://github.com/flutter/packages/labels/p%3A%20palette_generator) |
 | [path\_provider](./packages/path_provider/) | [![pub package](https://img.shields.io/pub/v/path_provider.svg)](https://pub.dev/packages/path_provider) | [![pub points](https://img.shields.io/pub/points/path_provider)](https://pub.dev/packages/path_provider/score) | [![popularity](https://img.shields.io/pub/popularity/path_provider)](https://pub.dev/packages/path_provider/score) | [![GitHub issues by-label](https://img.shields.io/github/issues/flutter/flutter/p%3A%20path_provider?label=)](https://github.com/flutter/flutter/labels/p%3A%20path_provider) | [![GitHub pull requests by-label](https://img.shields.io/github/issues-pr/flutter/packages/p%3A%20path_provider?label=)](https://github.com/flutter/packages/labels/p%3A%20path_provider) |
 | [pigeon](./packages/pigeon/) | [![pub package](https://img.shields.io/pub/v/pigeon.svg)](https://pub.dev/packages/pigeon) | [![pub points](https://img.shields.io/pub/points/pigeon)](https://pub.dev/packages/pigeon/score) | [![popularity](https://img.shields.io/pub/popularity/pigeon)](https://pub.dev/packages/pigeon/score) | [![GitHub issues by-label](https://img.shields.io/github/issues/flutter/flutter/p%3A%20pigeon?label=)](https://github.com/flutter/flutter/labels/p%3A%20pigeon) | [![GitHub pull requests by-label](https://img.shields.io/github/issues-pr/flutter/packages/p%3A%20pigeon?label=)](https://github.com/flutter/packages/labels/p%3A%20pigeon) |
-| [platform](./packages/platform/) | [![pub package](https://img.shields.io/pub/v/platform.svg)](https://pub.dev/packages/platform) | [![pub points](https://img.shields.io/pub/points/platform)](https://pub.dev/packages/platform/score) | [![popularity](https://img.shields.io/pub/popularity/platform)](https://pub.dev/packages/platform/score) | [![GitHub issues by-label](https://img.shields.io/github/issues/flutter/flutter/p%3A%20platform?label=)](https://github.com/flutter/flutter/labels/p%3A%20platform) | [![GitHub pull requests by-label](https://img.shields.io/github/issues-pr/flutter/packages/p%3A%20platform?label=)](https://github.com/flutter/packages/labels/p%3A%20platform) |
 | [pointer\_interceptor](./packages/pointer_interceptor/) | [![pub package](https://img.shields.io/pub/v/pointer_interceptor.svg)](https://pub.dev/packages/pointer_interceptor) | [![pub points](https://img.shields.io/pub/points/pointer_interceptor)](https://pub.dev/packages/pointer_interceptor/score) | [![popularity](https://img.shields.io/pub/popularity/pointer_interceptor)](https://pub.dev/packages/pointer_interceptor/score) | [![GitHub issues by-label](https://img.shields.io/github/issues/flutter/flutter/p%3A%20pointer_interceptor?label=)](https://github.com/flutter/flutter/labels/p%3A%20pointer_interceptor) | [![GitHub pull requests by-label](https://img.shields.io/github/issues-pr/flutter/packages/p%3A%20pointer_interceptor?label=)](https://github.com/flutter/packages/labels/p%3A%20pointer_interceptor) |
 | [plugin\_platform\_interface](./packages/plugin_platform_interface/) | [![pub package](https://img.shields.io/pub/v/plugin_platform_interface.svg)](https://pub.dev/packages/plugin_platform_interface) | [![pub points](https://img.shields.io/pub/points/plugin_platform_interface)](https://pub.dev/packages/plugin_platform_interface/score) | [![popularity](https://img.shields.io/pub/popularity/plugin_platform_interface)](https://pub.dev/packages/plugin_platform_interface/score) | [![GitHub issues by-label](https://img.shields.io/github/issues/flutter/flutter/p%3A%20plugin_platform_interface?label=)](https://github.com/flutter/flutter/labels/p%3A%20plugin_platform_interface) | [![GitHub pull requests by-label](https://img.shields.io/github/issues-pr/flutter/packages/p%3A%20plugin_platform_interface?label=)](https://github.com/flutter/packages/labels/p%3A%20plugin_platform_interface) |
 | [process](./packages/process/) | [![pub package](https://img.shields.io/pub/v/process.svg)](https://pub.dev/packages/process) | [![pub points](https://img.shields.io/pub/points/process)](https://pub.dev/packages/process/score) | [![popularity](https://img.shields.io/pub/popularity/process)](https://pub.dev/packages/process/score) | [![GitHub issues by-label](https://img.shields.io/github/issues/flutter/flutter/p%3A%20process?label=)](https://github.com/flutter/flutter/labels/p%3A%20process) | [![GitHub pull requests by-label](https://img.shields.io/github/issues-pr/flutter/packages/p%3A%20process?label=)](https://github.com/flutter/packages/labels/p%3A%20process) |

packages/platform/README.md

@@ -1,10 +1,2 @@
-[![Pub](https://img.shields.io/pub/v/platform.svg)](https://pub.dartlang.org/packages/platform)
-
-A generic platform abstraction for Dart.
-
-Like `dart:io`, `package:platform` supplies a rich, Dart-idiomatic API for
-accessing platform-specific information.
-
-`package:platform` provides a lightweight wrapper around the static `Platform`
-properties that exist in `dart:io`. However, it uses instance properties rather
-than static properties, making it possible to mock out in tests.
+The source code for `package:platform` has been moved to the
+Dart GitHub org: https://github.com/dart-lang/platform/