AST design

Guide to create abstract syntax tree

• create terminals at the latest node possible before it changes type -- e.g. IdentifierReference(Expression) to IDENTIFIER, IdentifierReference(Expression) will be the last point we create a node
• create a node if the node is changing type -- e.g. Expression to Statement
• avoid creating a new node if what it is doing is just type casting -- e.g. from NewExpression to MemberExpression

... more?