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

@@ -92,25 +92,34 @@ class ChannelCommand extends FlutterCommand {
   }
 
   static Future<Null> _checkout(String branchName) async {
+    // Get latest refs from upstream.
     int result = await runCommandAndStreamOutput(
-      <String>['git', 'show-ref', '--verify', '--quiet', 'refs/heads/$branchName'],
+      <String>['git', 'fetch'],
       workingDirectory: Cache.flutterRoot,
       prefix: 'git: ',
     );
+
     if (result == 0) {
-      // branch already exists, try just switching to it
-      result = await runCommandAndStreamOutput(
-        <String>['git', 'checkout', branchName],
-        workingDirectory: Cache.flutterRoot,
-        prefix: 'git: ',
-      );
-    } else {
-      // branch does not exist, we have to create it
       result = await runCommandAndStreamOutput(
-        <String>['git', 'checkout', '--track', '-b', branchName, 'origin/$branchName'],
+        <String>['git', 'show-ref', '--verify', '--quiet', 'refs/heads/$branchName'],
         workingDirectory: Cache.flutterRoot,
         prefix: 'git: ',
       );
+      if (result == 0) {
+        // branch already exists, try just switching to it
+        result = await runCommandAndStreamOutput(
+          <String>['git', 'checkout', branchName],
+          workingDirectory: Cache.flutterRoot,
+          prefix: 'git: ',
+        );
+      } else {
+        // branch does not exist, we have to create it
+        result = await runCommandAndStreamOutput(
+          <String>['git', 'checkout', '--track', '-b', branchName, 'origin/$branchName'],
+          workingDirectory: Cache.flutterRoot,
+          prefix: 'git: ',
+        );
+      }
     }
     if (result != 0)
       throwToolExit('Switching channels failed with error code $result.', exitCode: result);