Update documentation on `Isolate.removeOnExitListener` and others.

Update documentation on `Isolate.removeOnExitListener` and others.

Fixes issue #28535
BUG= http://dartbug.com/28535
R=floitsch@google.com

Committed: https://github.com/dart-lang/sdk/commit/b291f479bd758bc2ca7ec1b8f4cf0b5a515a1d95

[Lasse Reichstein Nielsen, 2017-01-27 10:29:21]

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

[floitsch, 2017-01-30 11:12:14]

LGTM.

https://codereview.chromium.org/2659873002/diff/1/sdk/lib/isolate/isolate.dart
File sdk/lib/isolate/isolate.dart (right):

https://codereview.chromium.org/2659873002/diff/1/sdk/lib/isolate/isolate.dar...
sdk/lib/isolate/isolate.dart:392: * If the isolate is already dead, no exit
message will be sent.
is already dead when?
When you add an exit listener?

[Lasse Reichstein Nielsen, 2017-01-30 12:08:14]

https://codereview.chromium.org/2659873002/diff/1/sdk/lib/isolate/isolate.dart
File sdk/lib/isolate/isolate.dart (right):

https://codereview.chromium.org/2659873002/diff/1/sdk/lib/isolate/isolate.dar...
sdk/lib/isolate/isolate.dart:392: * If the isolate is already dead, no exit
message will be sent.
Reworded:
 If the isolate has terminated before it can receive this request,
 no exit message will be sent.

[Lasse Reichstein Nielsen, 2017-01-30 12:08:42]

Description was changed from

==========
Update documentation on `Isolate.removeOnExitListener` and others.

Fixes issue #28535
BUG= http://dartbug.com/28535
==========

to

==========
Update documentation on `Isolate.removeOnExitListener` and others.

Fixes issue #28535
BUG= http://dartbug.com/28535
R=floitsch@google.com

Committed:
https://github.com/dart-lang/sdk/commit/b291f479bd758bc2ca7ec1b8f4cf0b5a515a1d95
==========

[Lasse Reichstein Nielsen, 2017-01-30 12:08:43]

Committed patchset #2 (id:20001) manually as
b291f479bd758bc2ca7ec1b8f4cf0b5a515a1d95 (presubmit successful).