Per isolate package root.

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 3dc0ba2f8d34ea1a9fbe74988cad0cafa2d16e82..eaaa3fbe4c25a8a02f95861c839489ef5036693f 100644
--- a/runtime/bin/isolate_data.h
+++ b/runtime/bin/isolate_data.h
@@ -20,15 +20,22 @@ class EventHandler;
 // when the isolate shuts down.
 class IsolateData {
  public:
-  explicit IsolateData(const char* url)
-      : script_url(strdup(url)), udp_receive_buffer(NULL) {
+  explicit IsolateData(const char* url, const char* package_root)
+      : script_url(strdup(url)),
+        package_root(NULL),
+        udp_receive_buffer(NULL) {
+    if (package_root != NULL) {
+      this->package_root = strdup(package_root);
+    }
   }
   ~IsolateData() {
     free(script_url);
+    free(package_root);
     free(udp_receive_buffer);
   }
 
   char* script_url;
+  char* package_root;
   uint8_t* udp_receive_buffer;
 
  private: