Make non-global constants have consistent naming (with just _ instead of _k) (#17584)

Our style guide says the k's are not necessary, and it seems like a good idea to make all the code be consistent on this.

Only naming changes to private vars: no logic changes.

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 cd4a1db..22c37f3 100644
--- a/packages/flutter_tools/lib/src/version.dart
+++ b/packages/flutter_tools/lib/src/version.dart
@@ -135,7 +135,7 @@
   /// In the absence of bugs and crashes a Flutter developer should never see
   /// this remote appear in their `git remote` list, but also if it happens to
   /// persist we do the proper clean-up for extra robustness.
-  static const String _kVersionCheckRemote = '__flutter_version_check__';
+  static const String _versionCheckRemote = '__flutter_version_check__';
 
   /// The date of the latest framework commit in the remote repository.
   ///
@@ -148,11 +148,11 @@
         'git',
         'remote',
         'add',
-        _kVersionCheckRemote,
+        _versionCheckRemote,
         'https://github.com/flutter/flutter.git',
       ]);
-      await _run(<String>['git', 'fetch', _kVersionCheckRemote, branch]);
-      return _latestGitCommitDate('$_kVersionCheckRemote/$branch');
+      await _run(<String>['git', 'fetch', _versionCheckRemote, branch]);
+      return _latestGitCommitDate('$_versionCheckRemote/$branch');
     } finally {
       await _removeVersionCheckRemoteIfExists();
     }
@@ -163,8 +163,8 @@
         .split('\n')
         .map((String name) => name.trim()) // to account for OS-specific line-breaks
         .toList();
-    if (remotes.contains(_kVersionCheckRemote))
-      await _run(<String>['git', 'remote', 'remove', _kVersionCheckRemote]);
+    if (remotes.contains(_versionCheckRemote))
+      await _run(<String>['git', 'remote', 'remove', _versionCheckRemote]);
   }
 
   static FlutterVersion get instance => context[FlutterVersion];