interpolation_visitor library Null safety


Given an interpolation, find all of its chunks, validate that they are only simple variable substitutions or else Intl.plural/gender calls, and keep track of the pieces of text so that other parts of the program can deal with the simple string sections and the generated parts separately. Note that this is a SimpleAstVisitor, so it only traverses one level of children rather than automatically recursing. If we find a plural or gender, which requires recursion, we do it with a separate special-purpose visitor.