Implement spawnUri from dart:isolate. This function allows us to

runtime/vm/object_store.h

Index: runtime/vm/object_store.h
===================================================================
--- runtime/vm/object_store.h	(revision 7714)
+++ runtime/vm/object_store.h	(working copy)
@@ -437,6 +437,11 @@
     root_library_ = value.raw();
   }
 
+  RawArray* import_map() const { return import_map_; }
+  void set_import_map(const Array& value) {
+    import_map_ = value.raw();
+  }
+
   // Returns head of list of registered libraries.
   RawLibrary* registered_libraries() const { return registered_libraries_; }
   void set_registered_libraries(const Library& value) {
@@ -566,6 +571,7 @@
   RawLibrary* native_wrappers_library_;
   RawLibrary* builtin_library_;
   RawLibrary* root_library_;
+  RawArray* import_map_;
   RawLibrary* registered_libraries_;
   RawGrowableObjectArray* pending_classes_;
   RawError* sticky_error_;