Typed AST: Migrate YIELD_EXPRESSION.
Refactor formal parameter list model to use it for arow function paramters.
Remove BINDING_IDENTIFIER fro the grammar.

In last verison of ECMAScript 6 specification draft,
the production where BINDING_IDENTIFIER can be "default" has been removed.
So no need to have a differenciation between BINDIG_IDENTIFIER
and IDENTIFIER_REFERENCE anymore.
Typed AST: Migrate IDENTIFIER_REFERENCE