Allow arity checks on dynamic function types

test/dart_codegen/expect/async/future.dart

 part of dart.async;
  abstract class Future<T> {static final _Future _nullFuture = ((__x10) => DEVC$RT.cast(__x10, DEVC$RT.type((Future<dynamic> _) {
   }
 ), DEVC$RT.type((_Future<dynamic> _) {
   }
 ), "CastExact", """line 98, column 38 of dart:async/future.dart: """, __x10 is _Future<dynamic>, true))(new Future.value(null));
  factory Future(computation()) {
   _Future result = new _Future<T>();
    Timer.run(() {
     try {
@@ skipping 141 matching lines @@
     if (!iterator.moveNext()) return false;
      return new Future.sync(() => f(iterator.current)).then((_) => true);
     }
   );
   }
  static Future doWhile(f()) {
   _Future doneSignal = new _Future();
    var nextIteration;
    nextIteration = Zone.current.bindUnaryCallback((bool keepGoing) {
     if (keepGoing) {
-      new Future.sync(f).then(DEVC$RT.cast(nextIteration, dynamic, __t12, "CastGeneral", """line 361, column 33 of dart:async/future.dart: """, nextIteration is __t12, false), onError: doneSignal._completeError);
+      new Future.sync(f).then(DEVC$RT.cast(nextIteration, dynamic, __t12, "CastGeneral", """line 361, column 33 of dart:async/future.dart: """, nextIteration is __t12, true), onError: doneSignal._completeError);
       }
      else {
       doneSignal._complete(null);
       }
     }
   , runGuarded: true);
    nextIteration(true);
    return doneSignal;
   }
  Future then(onValue(T value), {
@@ skipping 28 matching lines @@
  void _completeWithErrorCallback(_Future result, error, stackTrace) {
 AsyncError replacement = Zone.current.errorCallback(error, DEVC$RT.cast(stackTrace, dynamic, StackTrace, "CastGeneral", """line 719, column 62 of dart:async/future.dart: """, stackTrace is StackTrace, true));
  if (replacement != null) {
 error = _nonNullError(replacement.error);
  stackTrace = replacement.stackTrace;
 }
  result._completeError(error, DEVC$RT.cast(stackTrace, dynamic, StackTrace, "CastGeneral", """line 724, column 32 of dart:async/future.dart: """, stackTrace is StackTrace, true));
 }
  Object _nonNullError(Object error) => (error != null) ? error : new NullThrownError();
  typedef dynamic __t12(dynamic __u13);