function getPrefix(node) {
			if (node.computed) {
				if (node.optional) {
					return "?.[";
				}
				return "[";
			}
			if (node.optional) {
				return "?.";
			}
			return ".";
		}
Get the prefix of a given MemberExpression node.
If the MemberExpression node is a computed value it returns a
left bracket. If not it returns a period.
@param {ASTNode} node A MemberExpression node to get
@returns {string} The prefix of the node.