Fix path tracer test.

test/cctest/test-heap.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 4248 matching lines @@
   CHECK(heap->InOldPointerSpace(o->elements()));
   CHECK(heap->InOldPointerSpace(*o));
   Page* page = Page::FromAddress(o->elements()->address());
   CHECK(page->WasSwept() ||
         Marking::IsBlack(Marking::MarkBitFrom(o->elements())));
 }
 
 
 #ifdef DEBUG
 TEST(PathTracer) {
+  // Type cast checks fail because the path tracer abuses the map for marking.
+  i::FLAG_enable_slow_asserts = false;
   CcTest::InitializeVM();
   v8::HandleScope scope(CcTest::isolate());
 
   v8::Local<v8::Value> result = CompileRun("'abc'");
   Handle<Object> o = v8::Utils::OpenHandle(*result);
   CcTest::i_isolate()->heap()->TracePathToObject(*o);
 }
 #endif  // DEBUG