Add line handler to string stream

Add line handler to string stream

This adds the line handler to the string stream to provide callbacks
when a line is known to be available.

This change also refactors parts of the string stream to avoid
converting decoded char codes to a string before checking for line
break sequences. With this change the string decoder tracks all line
breaks.

There is still room for improvement as e.g. the coping of the list of
char codes when reading lines can be avoided by using anoher offset.

R=ager@google.com

BUG=
TEST=

Committed: https://code.google.com/p/dart/source/detail?r=2016

[Mads Ager (google), 2011-12-02 11:07:50]

lgtm