fix accidental var introductions

Follow-up from https://dart-review.googlesource.com/c/sdk/+/341084 where I accidentally went w/ a `var` style despite surrounding use of `final`.

Change-Id: I60202b032ee867989f876b8746effe83ec202bfd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/341104
Commit-Queue: Phil Quitslund <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>

Author: pq

pkg/analysis_server/lib/src/services/user_prompts/dart_fix_prompt_manager.dart

@@ -72,8 +72,8 @@ class DartFixPromptManager {
   Map<String, List<String?>> get currentContextSdkConstraints {
     final constraintMap = <String, List<String?>>{};
     for (final context in server.contextManager.analysisContexts) {
-      var workspace = context.contextRoot.workspace;
-      var sdkConstraints = workspace is PubWorkspace
+      final workspace = context.contextRoot.workspace;
+      final sdkConstraints = workspace is PubWorkspace
           ? workspace.pubPackages
               .map((p) => p.sdkVersionConstraint?.toString())
               .toList()