Use to_snapshot when cluster serializing LibraryPrefixes.

runtime/vm/object_service.cc

Index: runtime/vm/object_service.cc
diff --git a/runtime/vm/object_service.cc b/runtime/vm/object_service.cc
index baace752527f97a1292392981e207ac6dc7f0618..50c659ee44fdb8edfc9e029e0fd2362e2a53485b 100644
--- a/runtime/vm/object_service.cc
+++ b/runtime/vm/object_service.cc
@@ -568,19 +568,21 @@ void Library::PrintJSONImpl(JSONStream* stream, bool ref) const {
       if (entry.IsLibraryPrefix()) {
         prefix ^= entry.raw();
         ports = prefix.imports();
-        for (intptr_t i = 0; i < ports.Length(); i++) {
-          ns ^= ports.At(i);
-          if (ns.IsNull()) continue;
-
-          JSONObject jsdep(&jsarr);
-          jsdep.AddProperty("isDeferred", prefix.is_deferred_load());
-          jsdep.AddProperty("isExport", false);
-          jsdep.AddProperty("isImport", true);
-          prefixName = prefix.name();
-          ASSERT(!prefixName.IsNull());
-          jsdep.AddProperty("prefix", prefixName.ToCString());
-          target = ns.library();
-          jsdep.AddProperty("target", target);
+        if (!ports.IsNull()) {
+          for (intptr_t i = 0; i < ports.Length(); i++) {
+            ns ^= ports.At(i);
+            if (ns.IsNull()) continue;
+
+            JSONObject jsdep(&jsarr);
+            jsdep.AddProperty("isDeferred", prefix.is_deferred_load());
+            jsdep.AddProperty("isExport", false);
+            jsdep.AddProperty("isImport", true);
+            prefixName = prefix.name();
+            ASSERT(!prefixName.IsNull());
+            jsdep.AddProperty("prefix", prefixName.ToCString());
+            target = ns.library();
+            jsdep.AddProperty("target", target);
+          }
         }
       }
     }