Wait for concurrent sweeper in vm/cc/PrintJSON.

runtime/vm/object_test.cc

Index: runtime/vm/object_test.cc
===================================================================
--- runtime/vm/object_test.cc	(revision 41751)
+++ runtime/vm/object_test.cc	(working copy)
@@ -4133,6 +4133,16 @@
 TEST_CASE(PrintJSON) {
   Heap* heap = Isolate::Current()->heap();
   heap->CollectAllGarbage();
+  // We don't want to print garbage objects, so wait for concurrent sweeper.
+  // TODO(21620): Add heap iteration interface that excludes garbage (or
+  // use ObjectGraph).
+  PageSpace* old_space = heap->old_space();
+  {
+    MonitorLocker ml(old_space->tasks_lock());
+    while (old_space->tasks() > 0) {
+      ml.Wait();
+    }
+  }
   GrowableArray<Object*> objects;
   ObjectAccumulator acc(&objects);
   heap->IterateObjects(&acc);