This was fixed in #13150.

That commit rolled the engine to:
  flutter/engine@93b21795971357edbfd646f61b8d7f62388b0dae,

which rolled Dart to:
  dart-lang/sdk@70e5deacb54aea295665215837adaedd3d6a5bfa

which includes:
  dart-lang/sdk@d38c08d97358ebe4ee30e557bfee339dddda9b7a

Which fixes the breakage introduced in #13134.

This reverts commit f5d9c777.