Enforce Swift 6 strict concurrency during Swift compilation - #181654

  41 | @objc(FlutterLogger) public final class Logger: NSObject {
  42 |   private static var shared = Logger()
     |                      |- warning: static property 'shared' is not concurrency-safe because it is nonisolated global shared mutable state; this is an error in the Swift 6 language mode
     |                      |- note: convert 'shared' to a 'let' constant to make 'Sendable' shared state immutable
     |                      |- note: annotate 'shared' with '@MainActor' if property should only be accessed from the main actor
     |                      `- note: disable concurrency-safety checks if accesses are protected by an external synchronization mechanism

/Volumes/Work/s/w/ir/cache/builder/engine/src/flutter/shell/platform/darwin/common/SwiftTestingMain.swift:14:26: error: passing argument of non-sendable type 'SwiftTestingRunner' outside of main actor-isolated context may introduce data races; this is an error in the Swift 6 language mode
12 |   static func main() async {
13 |     let runner = SwiftTestingRunner()
14 |     let exitCode = await runner.run()
   |                          `- error: passing argument of non-sendable type 'SwiftTestingRunner' outside of main actor-isolated context may introduce data races; this is an error in the Swift 6 language mode
15 |     exit(exitCode)
16 |   }