Fast path constructor for Duration

dart2js doesn't like to inline the public Duration constructor, making
Duration arithmetic slow and bulky.

Ideally dart2js would figure out that the public constructor can be
inlined but we have a way to go before we could do that.

R=lrn@google.com

Review URL: https://codereview.chromium.org//717963002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41697 260f80e4-7a28-3924-810f-c04153c831b5
Revert "Fast path constructor for Duration"

Reverting due to accidentally included file.

TBR=lrn@google.com
BUG=

Review URL: https://codereview.chromium.org//719293002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41698 260f80e4-7a28-3924-810f-c04153c831b5
Redo "Fast path constructor for Duration"

TBR=lrn@google.com
BUG=

Review URL: https://codereview.chromium.org//715233006

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41699 260f80e4-7a28-3924-810f-c04153c831b5