Clean up FunctionDefinition in the IR S-expression representation.

Clean up FunctionDefinition in the IR S-expression representation.

Separate value parameters from the continuation for symmetry with
function invocations.  Do not make closure variables optional.  Do not
use '{' or '}', since it makes parsing more difficult and probably
screws up other tools that work with S-expressions.

Move closure variables after the parameters and just before the body,
since they are not part of the signature, do not appear at call sites,
and the implicit box creation happens at the beginning of the body
anyway.

R=asgerf@google.com, karlklose@google.com
BUG=

Committed: https://code.google.com/p/dart/source/detail?r=42395

[asgerf, 2014-12-16 14:17:32]

LGTM 

... except I'll have to counter-update the test cases when closure variables are
overhauled again :-(

[Kevin Millikin (Google), 2014-12-16 14:30:51]

Committed patchset #1 (id:1) manually as 42395 (presubmit successful).