Get rid of the polyfill identity set implementation in favour of a

Get rid of the polyfill identity set implementation in favour of a 
mock map that does linear search. At the moment, this is actually
competitively fast on the VM because hashCode for objects that don't
implement it is extremely slow. And it doesn't suffer from needing
to be updated when the core changes. Should still be removed once
there's identity map support.

BUG=

[Jennifer Messerly, 2012-12-17 19:29:52]

lgtm

https://chromiumcodereview.appspot.com/11567018/diff/2001/pkg/serialization/l...
File pkg/serialization/lib/src/serialization_helpers.dart (right):

https://chromiumcodereview.appspot.com/11567018/diff/2001/pkg/serialization/l...
pkg/serialization/lib/src/serialization_helpers.dart:239: keys = <K>[];
perhaps:

keys.clear();
values.clear(); ?

then keys/values can be final

[Alan Knight, 2012-12-17 21:15:26]

Committed as https://codereview.chromium.org/11613005

https://chromiumcodereview.appspot.com/11567018/diff/2001/pkg/serialization/l...
File pkg/serialization/lib/src/serialization_helpers.dart (right):

https://chromiumcodereview.appspot.com/11567018/diff/2001/pkg/serialization/l...
pkg/serialization/lib/src/serialization_helpers.dart:239: keys = <K>[];
On 2012/12/17 19:29:53, John Messerly wrote:
> perhaps:
> 
> keys.clear();
> values.clear(); ?
> 
> then keys/values can be final

Nice. Done.