Cache experimental natives sources as external strings.

test/cctest/test-serialize.cc

Index: test/cctest/test-serialize.cc
diff --git a/test/cctest/test-serialize.cc b/test/cctest/test-serialize.cc
index 4b76324cfa3a6075600239115d7bca29188e1727..1ed43ac338aa78b294b52412e33e9131bc0855b1 100644
--- a/test/cctest/test-serialize.cc
+++ b/test/cctest/test-serialize.cc
@@ -296,7 +296,7 @@ UNINITIALIZED_TEST(PartialSerialization) {
     {
       HandleScope scope(isolate);
       for (int i = 0; i < Natives::GetBuiltinsCount(); i++) {
-        isolate->bootstrapper()->NativesSourceLookup(i);
+        isolate->bootstrapper()->SourceLookup<Natives>(i);
       }
     }
     heap->CollectAllGarbage();
@@ -419,7 +419,7 @@ UNINITIALIZED_TEST(ContextSerialization) {
     {
       HandleScope scope(isolate);
       for (int i = 0; i < Natives::GetBuiltinsCount(); i++) {
-        isolate->bootstrapper()->NativesSourceLookup(i);
+        isolate->bootstrapper()->SourceLookup<Natives>(i);
       }
     }
     // If we don't do this then we end up with a stray root pointing at the
@@ -554,7 +554,7 @@ UNINITIALIZED_TEST(CustomContextSerialization) {
     {
       HandleScope scope(isolate);
       for (int i = 0; i < Natives::GetBuiltinsCount(); i++) {
-        isolate->bootstrapper()->NativesSourceLookup(i);
+        isolate->bootstrapper()->SourceLookup<Natives>(i);
       }
     }
     // If we don't do this then we end up with a stray root pointing at the