requiresFlutterImport property

Whether this rule only applies to files that import Flutter.

If true, the rule is skipped for files without package:flutter/ imports. Uses cached FileMetrics for O(1) lookup after first computation.

Example: Widget-specific rules:

@override
bool get requiresFlutterImport => true;

Impact: Skips ~300+ widget rules instantly for pure Dart files.

Default: false (runs on all files)