Attempt 2 -- Fix updating packages map on reload and add a regression test

runtime/bin/isolate_data.h

Index: runtime/bin/isolate_data.h
diff --git a/runtime/bin/isolate_data.h b/runtime/bin/isolate_data.h
index 792f8935bebe860926c0667e8141947d3bd24780..86059fb6b839b0978d22d93ce20d0167ab5a9d24 100644
--- a/runtime/bin/isolate_data.h
+++ b/runtime/bin/isolate_data.h
@@ -53,6 +53,14 @@ class IsolateData {
   char* packages_file;
   uint8_t* udp_receive_buffer;
 
+  void UpdatePackagesFile(const char* packages_file_) {
+    if (packages_file != NULL) {
+      free(packages_file);
+      packages_file = NULL;
+    }
+    packages_file = strdup(packages_file_);
+  }
+
   // While loading a loader is associated with the isolate.
   bool HasLoader() const { return loader_ != NULL; }
   Loader* loader() const {