Allow the VM to read Kernel files with external libraries

runtime/vm/kernel_reader.cc

Index: runtime/vm/kernel_reader.cc
diff --git a/runtime/vm/kernel_reader.cc b/runtime/vm/kernel_reader.cc
index bbb439a7267cdc866c32cb785b7d13c20521b14b..6a6904065a658097198026dfa6473cad47dcd5fd 100644
--- a/runtime/vm/kernel_reader.cc
+++ b/runtime/vm/kernel_reader.cc
@@ -253,6 +253,8 @@ void KernelReader::ReadLibrary(intptr_t kernel_offset) {
   LibraryHelper library_helper(&builder_);
   library_helper.ReadUntilIncluding(LibraryHelper::kCanonicalName);
   Library& library = LookupLibrary(library_helper.canonical_name_);
+  // The Kernel library is external implies that it is already loaded.
+  ASSERT(!library_helper.IsExternal() || library.Loaded());
   if (library.Loaded()) return;
 
   library_helper.ReadUntilIncluding(LibraryHelper::kName);