Fix tests to verify defaultArgListString specified as null are null.

Fix tests to verify defaultArgListString specified as null are null.

* introduces a sentinel default value to ensure that we know when asserting if `null` was specified or the result of an excluded optional arg

BUG=
R=brianwilkerson@google.com

Committed: https://github.com/dart-lang/sdk/commit/5a237935f08e283060e0985bd66ff9f09c342e3b

[pquitslund, 2017-03-24 17:56:35]

pquitslund@google.com changed reviewers:
+ brianwilkerson@google.com

[pquitslund, 2017-03-24 17:56:36]

It turns out that we only handle Lists when generating default args so the case
of a Map we discussed won't happen today.  That said, this fix ensures we'll
catch it and do the right thing if we do.  Also note that we only generate a
"null" default arg when completing cons/function/method invocations but will not
do so for arglists os I think that case is covered too.

[Brian Wilkerson, 2017-03-24 18:04:43]

lgtm

[pquitslund, 2017-03-24 18:53:08]

Description was changed from

==========
Fix tests to verify defaultArgListString specified as null are null.

* introduces a sentinel default value to ensure that we know when asserting if
`null` was specified or the result of an excluded optional arg

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

to

==========
Fix tests to verify defaultArgListString specified as null are null.

* introduces a sentinel default value to ensure that we know when asserting if
`null` was specified or the result of an excluded optional arg

BUG=
R=brianwilkerson@google.com

Committed:
https://github.com/dart-lang/sdk/commit/5a237935f08e283060e0985bd66ff9f09c342e3b
==========

[pquitslund, 2017-03-24 18:53:09]

Committed patchset #2 (id:20001) manually as
5a237935f08e283060e0985bd66ff9f09c342e3b (presubmit successful).