Improve performance by not doing binary searches.

Improve performance by not doing binary searches.

Normally getting the line and column of a SpanScanner or a
SourceLocation needs a binary search through all the line endings in the
source file. This CL avoids those searches by using an eager SpanScanner
which does extra computation to track its current line and column
position, and storing the line and column directly on _SimpleKey
objects.

See #12

R=rnystrom@google.com

Committed: https://github.com/dart-lang/yaml/commit/df28edda6846b052d75aea0585b8d8aa250d96f2

[Bob Nystrom, 2015-09-02 19:40:41]

lgtm

[nweiz, 2015-09-02 20:34:39]

Committed patchset #2 (id:20001) manually as
df28edda6846b052d75aea0585b8d8aa250d96f2 (presubmit successful).