Add summary tests for cascaded show/hide combinators.

Add summary tests for cascaded show/hide combinators.

At the moment these tests are mostly redundant, since the semantics of
cascaded combinators are implemented elsewhere.  However, when we add
a relinking mechanism these tests will ensure that cascaded combinator
semantics are properly implemented by the relinker.

R=brianwilkerson@google.com

Committed: https://github.com/dart-lang/sdk/commit/55983428b440fbdb4d434e3cbf675367b82b454e

[Paul Berry, 2015-11-17 18:31:14]

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

[Brian Wilkerson, 2015-11-17 18:37:32]

LGTM

[Paul Berry, 2015-11-17 22:02:22]

Committed patchset #1 (id:1) manually as
55983428b440fbdb4d434e3cbf675367b82b454e (presubmit successful).