Per isolate package root.

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index 0356d59c2f05076b0cf94ebb5545ca7f1c252081..47d144c05c4b65cfbaaf8cfd0d5157b4a283926f 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -815,6 +815,7 @@ class IsolateSpawnState {
                     const Instance& message);
   IsolateSpawnState(Dart_Port parent_port,
                     const char* script_url,
+                    const char* package_root,
                     const Instance& args,
                     const Instance& message);
   ~IsolateSpawnState();
@@ -824,6 +825,7 @@ class IsolateSpawnState {
 
   Dart_Port parent_port() const { return parent_port_; }
   char* script_url() const { return script_url_; }
+  char* package_root() const { return package_root_; }
   char* library_url() const { return library_url_; }
   char* class_name() const { return class_name_; }
   char* function_name() const { return function_name_; }
@@ -839,6 +841,7 @@ class IsolateSpawnState {
   Isolate* isolate_;
   Dart_Port parent_port_;
   char* script_url_;
+  char* package_root_;
   char* library_url_;
   char* class_name_;
   char* function_name_;