pkg/unittest: give Configuration a default (blank) implementation

pkg/unittest: give Configuration a default (blank) implementation

Use this to replace subclasses of SimpleConfiguration that mostly override members to replace behavior with no-op

BUG= https://code.google.com/p/dart/issues/detail?id=12495
R=gram@google.com

Committed: https://code.google.com/p/dart/source/detail?r=26316

[kevmoo-old, 2013-08-16 23:36:28]

With this change:

1) Folks who are currently subclassing Configuration are still broken (by
design). They should change to subclass SimpleConfiguration

2) Folks who want to do something super trivial w/ a Configuration (maybe
override only one member) can subclass Configuration and calling super.blank in
their ctor.

PTAL

[gram, 2013-08-19 16:15:00]

lgtm

[kevmoo-old, 2013-08-19 17:27:27]

Committed patchset #3 manually as r26316 (presubmit successful).