Do not emit type metadata for classes that are only emitted for rti.

Do not emit type metadata for classes that are only emitted for rti.

Classes only emitted for rti are empty shells and, even if they are
marked as reflectable, are not otherwise referenced from the program. So
we should not try to emit type metatdata because it is neither needed
for is it always complete (as most of the class, including its type
variables, is missing).

Fixes #24493.

BUG= http://dartbug.com/24493
R=sigmund@google.com

Committed: https://github.com/dart-lang/sdk/commit/80aa2a4cc0cb3a83833c74119f28d0c6ec53fed3

[herhut, 2015-10-07 13:12:12]

herhut@google.com changed reviewers:
+ sigmund@google.com

[Siggi Cherem (dart-lang), 2015-10-07 15:27:36]

lgtm

Question though: what is the namer's invariant that we are breaking by including
classes that are only for rti?

[Siggi Cherem (dart-lang), 2015-10-07 15:28:15]

and thanks for taking a look!

[herhut, 2015-10-07 15:36:00]

On 2015/10/07 15:28:15, Siggi Cherem (dart-lang) wrote:
> and thanks for taking a look!

The new namer does reference counting on metadata tokens to ensure that we only
actually emit those that are referenced somewhere in the program. The issue here
is that we reference a piece of metadata that was never actually declared (and
thus would not be available at runtime). This happens because we generate a
representation of the static types of a class' interfaces (which might have type
variables) without actually emitting that class (which would declare those type
variables). For example, for

class Point<X> implements OtherPoint<X>

we might generate a representation of the interface OtherPoint<X> which
references X but we do not actually fully emit Point<X>, so that reference
dangles.

The old namer did not do any reference counting and resolved dangling references
by just using some index -1 IIRC, which would fail at runtime.

[herhut, 2015-10-07 15:38:55]

Committed patchset #1 (id:1) manually as
80aa2a4cc0cb3a83833c74119f28d0c6ec53fed3 (presubmit successful).