This issue was originally filed by @seaneagan

It would be nice if [Iterable|Stream].expand's argument were an optional positional and defaulted to:

(x) => x is Iterable ? x : [x]

It could alternatively be recursive:

(x) => x is Iterable ? x.expand() : [x]

... but that could lead to cycles, and a more generic way to recursively walk an Iterable might be better:

void walk(f) => forEach((x) => x is Iterable ? x.walk(f) : f(x));

then you could just do:

var flattened = [];
iterable.walk(flattened.add);