Make Dart_Initialize fail if a VM built as a JIT runtime gets a precompiled snapshot or vice versa.

runtime/vm/dart.cc

Index: runtime/vm/dart.cc
diff --git a/runtime/vm/dart.cc b/runtime/vm/dart.cc
index f50a22b9e8a7aec85589081d5c8f54ef20ebdd81..d005ceee306f93553e07c9bde4dc37c532fddac2 100644
--- a/runtime/vm/dart.cc
+++ b/runtime/vm/dart.cc
@@ -131,6 +131,15 @@ const char* Dart::InitOnce(const uint8_t* vm_isolate_snapshot,
   if (vm_isolate_ != NULL || !Flags::Initialized()) {
     return "VM already initialized or flags not initialized.";
   }
+#if defined(DART_PRECOMPILED_RUNTIME)
+  if (instructions_snapshot == NULL) {
+    return "Precompiled runtime requires a precompiled snapshot";
+  }
+#else
+  if (instructions_snapshot != NULL) {
+    return "JIT runtime cannot run a precompiled snapshot";
+  }
+#endif
   set_thread_exit_callback(thread_exit);
   SetFileCallbacks(file_open, file_read, file_write, file_close);
   set_entropy_source_callback(entropy_source);