Makes switching channels remove version freshness stamp. (#21182)

When switching between channels, we were leaving around the version freshness stamp file (bin/cache/flutter_version_check.stamp), which meant that the flutter tool would read from that file to see what the cached date of the most recent commit to the current channel (branch) was. The problem was that since the file was created while on the previous channel, the cached date was for the wrong channel, so if you switch from master to beta, flutter would think that the channel was out of date, and a new version was available, at least for three days after the first time it checked (after three days since the last time the freshness was checked, the cached date would get updated).

This PR modifies the channel command to remove that stamp file whenever the user switches channels, so that the cached date will be from the right channel when it is recreated.

Fixes #21134

packages/flutter_tools/lib/src/version.dart

diff --git a/packages/flutter_tools/lib/src/version.dart b/packages/flutter_tools/lib/src/version.dart
index 45cd4a7..728d446 100644
--- a/packages/flutter_tools/lib/src/version.dart
+++ b/packages/flutter_tools/lib/src/version.dart
@@ -228,6 +228,22 @@
   @visibleForTesting
   static Duration timeToPauseToLetUserReadTheMessage = const Duration(seconds: 2);
 
+  /// Reset the version freshness information by removing the stamp file.
+  ///
+  /// New version freshness information will be regenerated when
+  /// [checkFlutterVersionFreshness] is called after this. This is typically
+  /// used when switching channels so that stale information from another
+  /// channel doesn't linger.
+  static Future<Null> resetFlutterVersionFreshnessCheck() async {
+    try {
+      await Cache.instance.getStampFileFor(
+        VersionCheckStamp.kFlutterVersionCheckStampFile,
+      ).delete();
+    } on FileSystemException {
+      // Ignore, since we don't mind if the file didn't exist in the first place.
+    }
+  }
+
   /// Checks if the currently installed version of Flutter is up-to-date, and
   /// warns the user if it isn't.
   ///