css pruning fails with /deep/ and ::shadow

[har79]

These selectors (including deep's alias >>>) are deprecated: https://code.google.com/p/chromium/issues/detail?id=489954
however there isn't a replacement yet and there are still discussions what form that will take so they'll likely be around for quite a while.

The bug appears to be in csslib; errors are of the form:
"error on line 33, column 17 of foo.css: expected {, but found /
foo /deep/ #bar {"
Message comes from https://github.com/dart-lang/csslib/blob/master/lib/parser.dart#L307
and forwarded by SCiSSors: https://github.com/google/dart-scissors/blob/master/lib/src/css_pruning/css_pruning.dart#L121

I've filed an issue with csslib (https://github.com/dart-lang/csslib/issues/23) and will work on a patch.