In CPS, closure variables are modeled as ref cells. In tree, variables
are in scope within inner functions and closure variables are handled directly.
In Dart, a function must be declared if it has a return type or is recursive.
Therefore tree now has both FunctionExpression and FunctionDeclaration, the latter
is mainly to be used for functions that cannot occur in expression context.
The CPS also has two ways of creating functions, CreateFunction and DeclareFunction,
but here the latter is only strictly necessary for recursive function.
The CL also contains quite a bit of other code improvements that came along.
BUG=
R=kmillikin@google.com, sigurdm@google.com
Review URL: https://codereview.chromium.org//366853007
git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@38005 260f80e4-7a28-3924-810f-c04153c831b5
asgerf@google.com