This is svn r6415 with a bugfix.

The bug was that we can have scopes with 0 heap slots (functions with
such scopes do not need a local context) and scopes with at least
Context::MIN_CONTEXT_SLOTS.  The code generator only allocated a local
context if there were strictly more than Context::MIN_CONTEXT_SLOTS.
Before the change to with/arguments, it was impossible to actually get
a function scope with exactly Context::MIN_CONTEXT_SLOTS because every
such scope included a heap-allocated arguments (and arguments shadow)
slot.

Review URL: http://codereview.chromium.org/6248014

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6426 ce2b1a6d-e550-0410-aec6-3dcde31c8c00