Note that this started failing with the experimental merge.

Related (I believe):

This throws a type check in dart2js as well:
new Future.immediate(true).then((bool complete) {});

While this does not:
new Future.immediate(true).then((complete) {});

This is showing up now because lib_v2 uses parameterized typedefs.

What is needed immediately is for checked mode type assertions to be 'optimistic'.
Longer term the assertions should be accurate.

Smaller example:

typedef T Transformation<S, T>(S value);

main() {
  Transformation<String, int> f;
  f = (x) => 123;
  f = (String s) => 321;
}

o.js:230: type 'main_anon0' is not a subtype of type 'Transformation'
  throw jsError;
        ^
type 'main_anon0' is not a subtype of type 'Transformation'
    at $.$$throw (o.js:225:13)
    at $.propertyTypeError (o.js:270:11)
    at $.propertyTypeCheck (o.js:278:5)
    at $.main (o.js:160:5)
    at o.js:378:7

$.main = function() {
  $.propertyTypeCheck(new $.main_anon(), "is$Transformation");
  $.propertyTypeCheck(new $.main_anon0(), "is$Transformation");
};

$$.main_anon = {"": "Closure;",
  call$1: function(x) {
    return 123;
  },
  is$Function: true,
  is$Transformation: true
};

$$.main_anon0 = {"": "Closure;",
  call$1: function(s) {
    $.stringTypeCheck(s);
    return 321;
  },
  is$Function: true // is$Transformation missing.
};

cc @karlklose.

Set owner to @karlklose.
Added Accepted label.

Removed this from the M3 milestone.
Added this to the M4 milestone.

Karl, is this still a critical unfixed issue?
If so, should it be blocking M4?

Johnni is working on function typechecks with type variables.

I don't think it should block M4 because the libraries do not currently use type variables.

cc @johnniwinther.

Removed this from the M4 milestone.
Added this to the M5 milestone.

Added TriageForM5 label.

Sounds like this is on Johnni's plate.

cc @lrhn.
Set owner to @johnniwinther.
Removed TriageForM5 label.

Removed Priority-Critical label.
Added Priority-High label.

Removed this from the M5 milestone.
Added this to the M6 milestone.

Fixed by https://codereview.chromium.org/12334070/.

Added Fixed label.