Reapply "Sharing of sources for corelib date implementation between VM

Reapply "Sharing of sources for corelib date implementation between VM
and dart2js".

BUG=

Committed: https://code.google.com/p/dart/source/detail?r=10799

[Mads Ager (google), 2012-08-16 07:57:41]

Only change is to ignore 'external' constructors in the same way as redirecting
constructors in the analyzer. That change is isolated in patch set 2.
Konstantin, could you double check that this makes sense.

[Mads Ager (google), 2012-08-16 07:59:18]

On 2012/08/16 07:57:41, Mads Ager wrote:
> Only change is to ignore 'external' constructors in the same way as
redirecting
> constructors in the analyzer. That change is isolated in patch set 2.
> Konstantin, could you double check that this makes sense.

And by ignoring I only mean when checking that all final fields are initialized.
We cannot check that for 'external' constructors since we don't have the
implementation.

[Anders Johnsen, 2012-08-16 07:59:28]

LGTM

[scheglov, 2012-08-16 12:29:03]

lgtm