Create a new sourcefile using a dummy filename and a given content

Property definitions

nitc $ SourceFile :: from_string
	# Create a new sourcefile using a dummy filename and a given content
	init from_string(filename: String, string: String) is
		nosuper
	do
		self.filename = filename
		self.string = string
		line_starts[0] = 0
	end
src/location.nit:37,2--44,4