Only reference constant root list entries in the snapshot.

test/cctest/test-heap.cc

Index: test/cctest/test-heap.cc
diff --git a/test/cctest/test-heap.cc b/test/cctest/test-heap.cc
index d7c24156a3d2aed5fad23b591898cf7e510f4902..5ddbc99fea932c4a65e6ac40946cc67da05541c6 100644
--- a/test/cctest/test-heap.cc
+++ b/test/cctest/test-heap.cc
@@ -5092,3 +5092,14 @@ TEST(PathTracer) {
   CcTest::i_isolate()->heap()->TracePathToObject(*o);
 }
 #endif  // DEBUG
+
+
+TEST(WritableVsImmortalRoots) {
+  for (int i = 0; i < Heap::kStrongRootListLength; ++i) {
+    Heap::RootListIndex root_index = static_cast<Heap::RootListIndex>(i);
+    bool writable = Heap::RootCanBeWrittenAfterInitialization(root_index);
+    bool immortal = Heap::RootIsImmortalImmovable(root_index);
+    // A root value can be writable, immortal, or neither, but not both.
+    CHECK(!immortal || !writable);
+  }
+}