tree bbbd99dbb1c59ce1d9ca217e2aeff24c3cc48ae5
parent ca7d545df8a26c6c4514cda96d0168c41edb934a
author Paul Berry <paulberry@google.com> 1527203275 +0000
committer commit-bot@chromium.org <commit-bot@chromium.org> 1527203275 +0000

Observatory strong mode fixes: make callback parameter types more general.

This CL addresses a code pattern where a method expects its parameter
to have a certain type, but that method is torn off and passed as a
callback to another method expecting its parameter type to be more
general.  For example:

    void f(int i) { ... }
    void g(void callback(Object o)) { ... }
    void h() {
      g(f); // Error: () -> int is not a subtype of () -> Object
    }

This is a strong mode error because the type system cannot guarantee
that the value pased to f will be an int.  The solution is to broaden
the type of the callback parameter so that it matches the type
expected for the callback.  In most cases, we insert an implicit
downcast (by reassigning the parameter to a local variable with the
expected type), which in Dart 2.0 semantics will result in a runtime
check (similar to what happens in Dart 1.0 checked mode).

Since the downcasts are implicit, the Dart 1.0 semantics are
unchanged, so this should be a safe change.

Change-Id: I9583ea194343b89b39305c9796cfad299a47943f
Reviewed-on: https://dart-review.googlesource.com/55907
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>