Make ObjectGraph wait for concurrent sweeper before using mark bits

runtime/vm/object_graph_test.cc

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "platform/assert.h"
 #include "vm/object_graph.h"
 #include "vm/unit_test.h"
 
 namespace dart {
 
@@ skipping 94 matching lines @@
       intptr_t empty_length = graph.RetainingPath(&c, Object::empty_array());
       Array& path = Array::Handle(Array::New(1, Heap::kNew));
       intptr_t one_length = graph.RetainingPath(&c, path);
       EXPECT_EQ(null_length, empty_length);
       EXPECT_EQ(null_length, one_length);
       EXPECT_LE(3, null_length);
     }
     {
       HANDLESCOPE(isolate);
       Array& path = Array::Handle(Array::New(6, Heap::kNew));
+      // Trigger a full GC to increase probability of concurrent tasks.
+      isolate->heap()->CollectAllGarbage();
       intptr_t length = graph.RetainingPath(&c, path);
       EXPECT_LE(3, length);
       Array& expected_c = Array::Handle();
       expected_c ^= path.At(0);
       // c is the first element in b.
       Smi& offset_from_parent = Smi::Handle();
       offset_from_parent ^= path.At(1);
       EXPECT_EQ(Array::element_offset(0),
                 offset_from_parent.Value() * kWordSize);
       Array& expected_b = Array::Handle();
       expected_b ^= path.At(2);
       // b is the element with index 10 in a.
       offset_from_parent ^= path.At(3);
       EXPECT_EQ(Array::element_offset(10),
                 offset_from_parent.Value() * kWordSize);
       Array& expected_a = Array::Handle();
       expected_a ^= path.At(4);
       EXPECT(expected_c.raw() == c.raw());
       EXPECT(expected_b.raw() == a.At(10));
       EXPECT(expected_a.raw() == a.raw());
     }
   }
 }
 
 }  // namespace dart