fix(swingset): disable GC for now

Until #2724 is implemented, we cannot safely allow Presences to be collected,
because we have WeakMaps that think they'll stick around. Disable collection
by putting all imports into the (strong) `exported` Set.

We'll remove this when WeakMap/WeakSet is updated to know the difference
between non-REACHABLE and non-RECOGNIZABLE.

packages/SwingSet/src/kernel/liveSlots.js

@@ -444,6 +444,12 @@ function build(
         assert.fail(X`unrecognized slot type '${type}'`);
       }
       slotToVal.set(slot, new WeakRef(val));
+      // TODO: until #2724 is implemented, we cannot actually release
+      // Presences, else WeakMaps would forget their entries. We disable GC
+      // by stashing everything in the (strong) `exported` Set. Note that
+      // test-liveslots.js test('dropImports') passes despite this, because
+      // it uses a fake WeakRef that doesn't care about the strong reference.
+      exported.add(val);
       if (type === 'object' || type === 'device') {
         // we don't dropImports on promises, to avoid interaction with retire
         droppedRegistry.register(val, slot);