The specification of cascade invocation just says that e..suffix is equivalent to (t){t.suffix; return t;}(e) .
The "t" variable isn't typed, so there should be no warning for, e.g.,
  "abc"..noMethodNamedThis()
Also, the return type of the function literal is dynamic, so the type of e is lost there too:
  ("abc"..toString()).noMethodNamedThis()
should also not give a warning.

Could the spec of cascade be rewritten to not erase the type, e.g.,:
  ... equivalent to (((T t) { t.suffix; return t;})(e) as T) where T is the static type of e.

Or, alternatively, give static typing rules for cascades explicitly:
  e..suffix
has the static type T where T is the static type of e, and gives a warning if e.suffix would give a warning.