Per isolate package root.

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index 74d0394a57ab2dae0801233c96f34af7cf807a54..0fba4e464bd14df8b971bf250e714bb3a9ad4219 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -1431,6 +1431,7 @@ IsolateSpawnState::IsolateSpawnState(Dart_Port parent_port,
     : isolate_(NULL),
       parent_port_(parent_port),
       script_url_(NULL),
+      package_root_(NULL),
       library_url_(NULL),
       class_name_(NULL),
       function_name_(NULL),
@@ -1458,10 +1459,12 @@ IsolateSpawnState::IsolateSpawnState(Dart_Port parent_port,
 
 IsolateSpawnState::IsolateSpawnState(Dart_Port parent_port,
                                      const char* script_url,
+                                     const char* package_root,
                                      const Instance& args,
                                      const Instance& message)
     : isolate_(NULL),
       parent_port_(parent_port),
+      package_root_(NULL),
       library_url_(NULL),
       class_name_(NULL),
       function_name_(NULL),
@@ -1471,6 +1474,9 @@ IsolateSpawnState::IsolateSpawnState(Dart_Port parent_port,
       serialized_message_(NULL),
       serialized_message_len_(0) {
   script_url_ = strdup(script_url);
+  if (package_root != NULL) {
+    package_root_ = strdup(package_root);
+  }
   library_url_ = NULL;
   function_name_ = strdup("main");
   exception_callback_name_ = strdup("_unhandledExceptionCallback");
@@ -1481,6 +1487,7 @@ IsolateSpawnState::IsolateSpawnState(Dart_Port parent_port,
 
 IsolateSpawnState::~IsolateSpawnState() {
   free(script_url_);
+  free(package_root_);
   free(library_url_);
   free(function_name_);
   free(class_name_);