Futures API should check callback function types synchronously

[zanderso]

When the type of an error callback passed to the Futures API is incorrect, the error is flagged asynchronously, which obscures the location of the callback that had the wrong type, e.g.:

[1369.151] 425084.425899> [ERROR:third_party/flutter/shell/common/shell.cc(178)] Dart Error: Unhandled exception:
[1369.151] 425084.425899> type '(Error, StackTrace) => Null' is not a subtype of type '(Object) => dynamic'
[1369.151] 425084.425899> #0      _FutureListener.handleError (dart:async/future_impl.dart:144:46)
[1369.151] 425084.425899> #1      Future._propagateToListeners.handleError (dart:async/future_impl.dart:651:47)
[1369.151] 425084.425899> #2      Future._propagateToListeners (dart:async/future_impl.dart:672:24)
[1369.151] 425084.425899> #3      Future._completeError (dart:async/future_impl.dart:491:5)
[1369.151] 425084.425899> #4      Future.wait.<anonymous closure> (dart:async/future.dart:410:22)
[1369.151] 425084.425899> #5      _RootZone.runUnary (dart:async/zone.dart:1379:54)
[1369.151] 425084.425899> #6      _FutureListener.handleValue (dart:async/future_impl.dart:126:18)
[1369.151] 425084.425899> #7      Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:639:45)
[1369.151] 425084.425899> #8      Future._propagateToListeners (dart:async/future_impl.dart:668:32)
[1369.151] 425084.425899> #9      Future._complete (dart:async/future_impl.dart:473:7)
[1369.151] 425084.425899> #10     _SyncCompleter.complete (dart:async/future_impl.dart:51:12)
[1369.151] 425084.425899> #11     _AsyncAwaitCompleter.complete.<anonymous closure> (dart:async/runtime/libasync_patch.dart:33:20)
[1369.151] 425084.425899> #12     _microtaskLoop (dart:async/schedule_microtask.dart:41:21)
[1369.151] 425084.425899> #13     _startMicrotaskLoop (dart:async/schedule_microtask.dart:50:5)

Either the Futures API should check the callback types synchronously, or the analyzer should detect this problem statically.

Fuchsia issue number is FL-173.

/cc @lrhn @leafpetersen @bwilkerson @alexmarkov

[stereotype441]

Regardless of whether we add a runtime check, it makes sense for the analyzer to detect cases that will be known to fail. I've separated that out to its own issue: #35825