Use 16 instead of 4 for determining member offsets.

src/heap/heap-inl.h

Index: src/heap/heap-inl.h
diff --git a/src/heap/heap-inl.h b/src/heap/heap-inl.h
index 48e928d71106c1f7785b04d4cc7eebb8bd1b3de8..549ecbc9a64eafe8f93c77c67e7e90d1f6a9afe2 100644
--- a/src/heap/heap-inl.h
+++ b/src/heap/heap-inl.h
@@ -587,7 +587,7 @@ bool Heap::CollectGarbage(AllocationSpace space, const char* gc_reason,
 Isolate* Heap::isolate() {
   return reinterpret_cast<Isolate*>(
       reinterpret_cast<intptr_t>(this) -
-      reinterpret_cast<size_t>(reinterpret_cast<Isolate*>(4)->heap()) + 4);
+      reinterpret_cast<size_t>(reinterpret_cast<Isolate*>(16)->heap()) + 16);
 }