dart2js cps: Change type tests to null tests when applicable.

dart2js cps: Change type tests to null tests when applicable.

When null is the only value that might fail a type test we can
rewrite it to just compare against null.

Update: Fixed a missing mandatory named parameter.

R=kmillikin@google.com, sra@google.com

Committed: https://github.com/dart-lang/sdk/commit/29dedfa97e3ff513d93b89c69c6b42b9746925d6

Reverted: https://github.com/dart-lang/sdk/commit/e97d32be50504c965b2f598f7f833b54be066fdf

Committed: https://github.com/dart-lang/sdk/commit/cd901e78136644983e5933451248a53ac23a149b

[asgerf, 2015-09-03 18:34:57]

asgerf@google.com changed reviewers:
+ kmillikin@google.com, sra@google.com

[Kevin Millikin (Google), 2015-09-04 08:08:05]

lgtm

[sra1, 2015-09-09 17:47:50]

lgtm

[asgerf, 2015-09-21 11:33:21]

Committed patchset #1 (id:1) manually as
29dedfa97e3ff513d93b89c69c6b42b9746925d6 (presubmit successful).

[asgerf, 2015-09-21 12:25:12]

Patchset #2 (id:20001) has been deleted

[asgerf, 2015-09-21 13:41:26]

PTAL.

I was missing a mandatory named parameter "allowNull".

The naming of the parameter is unfortunate at this use-site, because we're
specifically passing in a null abstract value while saying "do not allow null".

I think the documentation of isSubtypeOf clarifies it, and I can't think of a
name that doesn't look confusing at all the other call sites, but I'm open to
ideas.

[asgerf, 2015-10-14 09:35:49]

Committed patchset #4 (id:80001) manually as
cd901e78136644983e5933451248a53ac23a149b (presubmit successful).