Request the presence of a global declaration

Property definitions

nitc $ AbstractCompilerVisitor :: require_declaration
	# Request the presence of a global declaration
	fun require_declaration(key: String)
	do
		var reqs = self.writer.file.required_declarations
		if reqs.has(key) then return
		reqs.add(key)
		var node = current_node
		if node != null then compiler.requirers_of_declarations[key] = node
	end
src/compiler/abstract_compiler.nit:1861,2--1869,4