[dart2wasm] No longer pass --experimental-wasm-imported-strings to V8 (js-string builtins are enabled by default)

Change-Id: I353e3dbd693d8ce83f75059a2d6dd72b277d3677
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/406842
Commit-Queue: Martin Kustermann <kustermann@google.com>
Reviewed-by: Ömer Ağacan <omersa@google.com>

pkg/test_runner/lib/src/browser_controller.dart

@@ -410,7 +410,6 @@ class Chrome extends Browser {
         "--use-mock-keychain",
         "--password-store=basic",
         "--user-data-dir=${userDir.path}",
-        "--js-flags=--experimental-wasm-imported-strings",
         url,
       ];