Avoid using SubListIterable for _JsonMap.keys.

BUG=
R=floitsch@google.com, kasperl@google.com

Review URL: https://codereview.chromium.org//794483003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42280 260f80e4-7a28-3924-810f-c04153c831b5
Don't merge indirect entries.

R=fschneider@google.com
BUG=

Review URL: https://codereview.chromium.org//792023002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42281 260f80e4-7a28-3924-810f-c04153c831b5