add missing type parameter on methods (#22096)

diff --git a/packages/flutter_tools/lib/src/version.dart b/packages/flutter_tools/lib/src/version.dart
index e1dea0e..98612c0 100644
--- a/packages/flutter_tools/lib/src/version.dart
+++ b/packages/flutter_tools/lib/src/version.dart
@@ -161,7 +161,7 @@
   static Future<Null> _removeVersionCheckRemoteIfExists() async {
     final List<String> remotes = (await _run(<String>['git', 'remote']))
         .split('\n')
-        .map((String name) => name.trim()) // to account for OS-specific line-breaks
+        .map<String>((String name) => name.trim()) // to account for OS-specific line-breaks
         .toList();
     if (remotes.contains(_versionCheckRemote))
       await _run(<String>['git', 'remote', 'remove', _versionCheckRemote]);