Constant fold codeUnitAt via constant system

Constant fold charCodeAt via constant system

BUG=
R=johnniwinther@google.com

Committed: https://code.google.com/p/dart/source/detail?r=41381

[sra1, 2014-10-25 03:37:57]

sra@google.com changed reviewers:
+ johnniwinther@google.com

[sra1, 2014-10-25 03:37:58]

I wanted to constant-fold 'a'.codeUnitAt(0)

It seemed that the ConstantSystem was a good place to put the logic. I found
that Operations do not cleanly support constant-folding of instance methods (and
static methods are probably worse).

The main wart at this point is BinaryOperation.apply.
I believe apply is used by range analysis to do operations on symbolic ranges.

[Johnni Winther, 2014-10-27 09:44:48]

Could you add a unit test for this feature?

[sra1, 2014-10-27 22:53:12]

PTAL

[Johnni Winther, 2014-10-28 07:37:48]

LGTM

Would be nice with a language test that ''.codeUnitAt(0) is not a compile time
constant.

[sra1, 2014-10-28 23:58:21]

Committed patchset #4 (id:60001) manually as 41381 (presubmit successful).