Remove special list behavior in analysis server protocol.

Remove special list behavior in analysis server protocol.

Previously, for optional lists, the absence of a list was treated by
the toJson and fromJson methods as equivalent to the presence of an
empty list.  This disagreed with the API documentation in several
places and created the risk of null reference exceptions in clients.

This change makes optional lists behave just like optional fields of
any other type.

BUG=dartbug.com/21230
R=brianwilkerson@google.com

Committed: https://code.google.com/p/dart/source/detail?r=40939

[Paul Berry, 2014-10-06 02:12:57]

paulberry@google.com changed reviewers:
+ brianwilkerson@google.com, danrubel@google.com

[Brian Wilkerson, 2014-10-06 14:04:07]

LGTM

[Paul Berry, 2014-10-06 16:32:24]

Committed patchset #1 (id:1) manually as 40939 (presubmit successful).