Fix order of operations dependencies in element_references_test

Fix order of operations dependencies in element_references_test

It's not safe for a single test to have two overlapping calls to
findElementReferences() because fields in the test class are used to
hold the search results and to keep track of whether more results are
pending.

Fixes a bogus test failure that arose after the relative ordering of
futures and microtasks was changed.

BUG=dartbug.com/22014
R=scheglov@google.com

Committed: https://code.google.com/p/dart/source/detail?r=42738

[Paul Berry, 2015-01-08 19:48:45]

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

[scheglov, 2015-01-08 19:50:44]

LGTM

Thank you!

[Paul Berry, 2015-01-09 16:36:48]

Committed patchset #1 (id:1) manually as 42738 (presubmit successful).