Add hint for incorrect assignment based on propagated types.

[stereotype441]

This code:

  class A {}
  class B {}
  main() {
    Map<String, A> c = <String, A>{};
    c['foo'] = new B();
  }

produces the warning "A value of type 'B' cannot be assigned to a variable of type 'A'", as expected.

This code should produce the equivalent hint, but it doesn't:

  class A {}
  class B {}
  main() {
    var c = <String, A>{};
    c['foo'] = new B();
  }

[bwilkerson]

Added this to the Later milestone.
Removed Type-Defect, Priority-Unassigned labels.
Added Type-Enhancement, Priority-Medium labels.

[kasperl]

Removed this from the Later milestone.
Added Oldschool-Milestone-Later label.

[bwilkerson]

Here is another concrete example:

main() {
  List<int> x = [0.5];
}

While the right-hand side has a static type of List<dynamic>, which is assignable to List<int>, it has a propagated type of List<double>, which isn't assignable.

---

Set owner to collinsn@google.com.
Removed Oldschool-Milestone-Later label.
Changed the title to: "Add hint for incorrect assignment based on propagated types.".

[DartBot]

This comment was originally written by collinsn@google.com

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My internship is ending today.

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Set owner to @bwilkerson.

[srawlins]

This works with Strong Mode. https://dartpad.dartlang.org/2df2687e6c8032763f26d8af5e53b213