Fix least upper bound computation for generic types (dartbug.com/15060)

Fix least upper bound computation for generic types (dartbug.com/15060)

Consider the classes:

class A<T> {}
class B<T> extends A<T>
class C<T> extends A<T>
class D {}
class E {}

When computing the least upper bound of B<D> and C<E>, we were making
two mistakes:

1. When computing superclasses we were using
ClassElement.getSupertype() (which doesn't substitute type arguments),
so B<D> and C<E> were considered to derive from a common type A<T>
rather than distinct types A<D> and A<E>.  A similar problem existed
for interfaces.

2. When intersecting superinterface sets, we were recursively taking
the least upper bound of type arguments, so A<D> and A<E> were
coalesced into A<Object>.  The spec and VM consider A<D> and A<E> to
be unrelated types for the purpose of least upper bound computation,
so the least upper bound should simply be Object.

R=brianwilkerson@google.com

Committed: https://code.google.com/p/dart/source/detail?r=34952

[Brian Wilkerson, 2014-04-10 22:54:51]

LGTM

[Paul Berry, 2014-04-10 23:07:56]

Committed patchset #1 manually as r34952 (presubmit successful).