Description:
------------
While verifying the signature of a class against the signature of an abstract class it extends, apparently class aliases are not correctly taken into account. This works if the class implements an interface or extends a non-abstract class.

See also https://github.com/facebook/hhvm/issues/2527.

Test script:
---------------
class A {}

class_alias( 'A', 'B' );

abstract class UserOfA {
  abstract public function someMethod( B $a );
}

class ActualUserOfA extends UserOfA {
  public function someMethod( A $a ) {
  }
}


Actual result:
--------------
PHP Fatal error:  Declaration of ActualUserOfA::someMethod() must be compatible with UserOfA::someMethod(B $a)