Fix handling of nested typedefs (for real this time).

Fix handling of nested typedefs (for real this time).

This is a re-fix of dartbug.com/21912, which I previously fixed
incorrectly.  Previously, our approach to avoiding infinite loops when
comparing types was to maintain a set of typedefs being expanded on
the stack, and prune the comparison whenever an attempt was made to
expand a typedef that was already being expanded.  However, this was
too strict, since there are legal (non-circular) types which invole
expanding a given typedef in reentrant fashion; we can't prune these
types without producing incorrect semantics.  An example (from the bug
report) is the type of f in the code below:

    typedef T Function2<S, T>(S z);
    Function2<Function2<A, B>, Function2<B, A>> f;

The solution is to maintain the list of typedefs being expanded inside
each FunctionTypeImpl object (and InterfaceTypeImpl object) rather
than on the stack during the comparison; this allows us to distinguish
the situations where we need to prune (those having to do exclusively
with expansion of a typedef) from the situations where we shouldn't
prune (those having to do with substitution of a type parameter).

A beneficial side effect of this change is that code that interacts
with types no longer needs to worry about typedef circularities, since
the circularities will automatically be pruned while exploring the
type definitions.  This simplifies the implementation of
isAssignableTo, isSubtypeOf, operator==, and hashCode.  (Note,
however, that code still needs to cope with circularities in the
inheritance hierarchy).

BUG=dartbug.com/21912
R=brianwilkerson@google.com

Committed: https://github.com/dart-lang/sdk/commit/62f7ac8c5db16a22ac901f8639a80b532d230376

[Paul Berry, 2015-06-01 18:08:50]

paulberry@google.com changed reviewers:
+ brianwilkerson@google.com

[Brian Wilkerson, 2015-06-01 19:00:40]

LGTM

[Paul Berry, 2015-06-01 19:45:33]

Committed patchset #1 (id:1) manually as
62f7ac8c5db16a22ac901f8639a80b532d230376 (presubmit successful).