Fix call to `git checkout` (#15026)

In the case of "dev", `git checkout dev` could be miconstrued
as checking out the dev _directory_ rather than the branch...

https://github.com/flutter/flutter/issues/14865

packages/flutter_tools/lib/src/commands/channel.dart

diff --git a/packages/flutter_tools/lib/src/commands/channel.dart b/packages/flutter_tools/lib/src/commands/channel.dart
index d47381e..42b8fe4 100644
--- a/packages/flutter_tools/lib/src/commands/channel.dart
+++ b/packages/flutter_tools/lib/src/commands/channel.dart
@@ -108,7 +108,7 @@
       if (result == 0) {
         // branch already exists, try just switching to it
         result = await runCommandAndStreamOutput(
-          <String>['git', 'checkout', branchName],
+          <String>['git', 'checkout', branchName, '--'],
           workingDirectory: Cache.flutterRoot,
           prefix: 'git: ',
         );