Tweak formatting rules. Fix #211.

Tweak formatting rules. Fix #211.

Discard open spans when a hard split occurs.

This is a correctness issue, not a performance one. The previous
code would leave spans open even when a hard split occurred in
them. These were mostly innocuous since they ended up with
totally bogus ranges, but they apparently do affect some
corner-case formatting.

Some of this was triggered by the previous patch which was for
perf but also made some stuff more correct.

BUG=https://github.com/dart-lang/dart_style/issues/
R=pquitslund@google.com

Committed: 425ed080df27a2f66ab91f7161bb0600e4329ea6

[Bob Nystrom, 2015-03-10 22:59:29]

rnystrom@google.com changed reviewers:
+ pquitslund@google.com

[Bob Nystrom, 2015-03-10 22:59:30]

It turns out that the old behavior I changed in
https://github.com/dart-lang/dart_style/commit/ff182727c4b49f55a829645468b63d...
was masking some weird bugs in the formatter.

I also noticed another mostly innocuous bug where we'd keep old spans around
even after a hard split occurred, which was totally bogus.

Fixing those bugs caused the formatted output to change, sometimes for the worse
(the bugs conveniently made nicer output). This fixes the span issue and also
tweaks some of the formatting rules to get the output in better shape too.

[pquitslund, 2015-03-10 23:34:54]

lgtm

[Bob Nystrom, 2015-03-10 23:46:20]

Committed patchset #2 (id:20001) manually as
425ed080df27a2f66ab91f7161bb0600e4329ea6 (presubmit successful).