Use Null argument for callback arity check in async_error

Use Null argument for callback arity check in async_error.

Avoid relying on fuzzy arrow semantics by using Null for the callback type
parameter to be checked against.  Any function of the right arity will pass the "is"
check.

BUG=
R=lrn@google.com

Committed: https://github.com/dart-lang/sdk/commit/8cdcfb98e2641f6043259309e8fa9157356c842d

[Leaf, 2017-05-11 20:01:09]

Description was changed from

==========
Use Null argument for callback arity check in async_error

BUG=
==========

to

==========
Use Null argument for callback arity check in async_error.

Avoid relying on fuzzy arrow semantics by using Null for the callback type 
parameter to be checked against.  Any function of the right arity will pass the
"is"
check.

BUG=
==========

[Leaf, 2017-05-11 20:01:22]

leafp@google.com changed reviewers:
+ lrn@google.com

[Lasse Reichstein Nielsen, 2017-05-11 22:19:23]

lgtm

[Leaf, 2017-05-12 00:36:58]

Description was changed from

==========
Use Null argument for callback arity check in async_error.

Avoid relying on fuzzy arrow semantics by using Null for the callback type 
parameter to be checked against.  Any function of the right arity will pass the
"is"
check.

BUG=
==========

to

==========
Use Null argument for callback arity check in async_error.

Avoid relying on fuzzy arrow semantics by using Null for the callback type
parameter to be checked against.  Any function of the right arity will pass the
"is"
check.

BUG=
R=lrn@google.com

Committed:
https://github.com/dart-lang/sdk/commit/8cdcfb98e2641f6043259309e8fa9157356c842d
==========

[Leaf, 2017-05-12 00:36:59]

Committed patchset #1 (id:1) manually as
8cdcfb98e2641f6043259309e8fa9157356c842d (presubmit successful).