Strong assignment doesn't update fields

Consider the following example from the nullness checker:

import org.checkerframework.checker.nullness.qual.*;

class ObjectAssignment {

   @Nullable Object o;

   static void test_nullness(ObjectAssignment a, ObjectAssignment b) {
	a.o = new Object();
	b = a;
	int k = b.o.hashCode(); // issues a warning
	int j = a.o.hashCode(); // does not issue a warning
    }
}

This also occurs in other checkers (e.g. Index). In general, if an object a with field f is assigned (i.e. a = b), then a.f should have the same type as b.f.

Start by reproducing the nullness-checker example from the issue, then inspect how assignments are handled in the nullness and Index checkers. Done means assigning one object to another updates the selected object's field types consistently, so the example and analogous Index cases produce the expected warnings.