linter: Simplify lint rule selection

Just a little nit; it took me a second to figure out what was going on
with the `add` tearoff.

Change-Id: I96a59ea9fe513fa28894c0cb41ede7a07dbb18b7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/404521
Reviewed-by: Phil Quitslund <[email protected]>
Commit-Queue: Samuel Rawlins <[email protected]>

pkg/linter/tool/checks/driver.dart

@@ -86,8 +86,7 @@ class Driver {
             (context as DriverBasedAnalysisContext).allAnalysisOptions;
         for (var options in allOptions) {
           options as AnalysisOptionsImpl;
-          options.lintRules = options.lintRules.toList();
-          lints.forEach(options.lintRules.add);
+          options.lintRules = [...options.lintRules, ...lints];
           options.lint = true;
         }