Fixing bug in the evaluation order of watchers

Review URL: https://chromiumcodereview.appspot.com//12162005
Update CHANGELOG.md
keep track of pending paths and ignore dupicates to fix #336

_pending shoud not be cleared until the flushed Futures complete; and we
check _pending from writeString() before setting up an
open->write->close future chain on it.  This solves the case of multiple
parallel writes to the same file with the same contents.
we remove() from _pending in the continuation after we close() the file

this means we don't need to clear _pending from flush(); in flush() we
do a toList() on the values Iterable to guarantee we won't be removing
from our active iterator
Merge pull request #345 from rmsmith/master

keep track of pending paths and ignore duplicates to fix #336
Use whenComplete so the condition runs even in error case.

Review URL: https://chromiumcodereview.appspot.com//12181020
Fixes for next sdk

Review URL: https://chromiumcodereview.appspot.com//12181004
Print unit in the output of perf.dart

Review URL: https://chromiumcodereview.appspot.com//12186027
Add sdk dependency to pubspec

Review URL: https://chromiumcodereview.appspot.com//12208015
include revision in sdk constraint
Added CSS polyfill

Mutation problem

minor cleanup

make CSS polyfill optional --css_polyfill

changes to match updated csslib.

integrate CR comments

comment cleanup

cleaned a few func

cleanup

integrated CL comments

Added CSS polyfill

Mutation problem

minor cleanup

make CSS polyfill optional --css_polyfill

changes to match updated csslib.

integrate CR comments

comment cleanup

Integrated CL comments

Integrated CL comments

Integrated CL comments

Chnage to set

Integrated CL comments

Updated readme

re-added flag
Integrated more CR comment
cleanup