Allow sub-class methods to make superclass method parameters optional.

[lrhn]

If you have a class A:
 class A{
  foo(int a, int b) {}
 }
please make it possible to specialize this by making some parameters optional, e.g.:
 class B extends A {
  foo(int a, [int bar]) {}
 }
(ditto for interfaces).

This is a safe generalization of the superclass interface, since all calls to the superclass method are still valid for the subclass.

It also allows a unified implementation of different interfaces, e.g.,
  interface I {
    foo(int a, int b);
  }
  interface J {
    foo(String v);
  }
  class Unifier implements I,J {
    foo(Object x, [int bar]) {]
  }
This is currently not possible, because for one function type to be a subtype of another, they need to have the same number of non-optional arguments.

[DartBot]

This comment was originally written by @seaneagan

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this should be merged with issue #1827 or issue #2706

[gbracha]

Added Duplicate label.
Marked as being merged into #2706.