Fetch upstream refs before checking out branch in ChannelCommand (#14896)

https://github.com/flutter/flutter/issues/14893

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

@@ -92,7 +92,15 @@ class ChannelCommand extends FlutterCommand {
   }
 
   static Future<Null> _checkout(String branchName) async {
+    // Get latest refs from upstream.
     int result = await runCommandAndStreamOutput(
+      <String>['git', 'fetch'],
+      workingDirectory: Cache.flutterRoot,
+      prefix: 'git: ',
+    );
+
+    if (result == 0) {
+      result = await runCommandAndStreamOutput(
         <String>['git', 'show-ref', '--verify', '--quiet', 'refs/heads/$branchName'],
         workingDirectory: Cache.flutterRoot,
         prefix: 'git: ',
@@ -112,6 +120,7 @@ class ChannelCommand extends FlutterCommand {
           prefix: 'git: ',
         );
       }
+    }
     if (result != 0)
       throwToolExit('Switching channels failed with error code $result.', exitCode: result);
   }