dart2dart: Remove the list of closureVariables from FunctionDefinitions.

dart2dart: Remove the list of closureVariables from FunctionDefinitions.

Function definitions had a list of closure variables (which was always
empty in the JS backend).  This list has no code generation semantics
because it is ignored by the Tree IR builder.  Instead, when translating
to Tree IR, a reference to an unbound closure variable implicitly
'declares' it.

The closure variables were however used for the S-expression serialized
representation.  They can be eliminated here as well by using the same
trick as the Tree IR builder.

R=asgerf@google.com
BUG=

Committed: https://code.google.com/p/dart/source/detail?r=43337

[asgerf, 2015-01-30 10:58:14]

Much better! LGTM

[Kevin Millikin (Google), 2015-01-30 13:20:38]

Committed patchset #2 (id:20001) manually as 43337 (presubmit successful).