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

runtime/vm/object.h

Index: runtime/vm/object.h
===================================================================
--- runtime/vm/object.h	(revision 7517)
+++ runtime/vm/object.h	(working copy)
@@ -1702,8 +1702,6 @@
 
   RawString* url() const { return raw_ptr()->url_; }
   RawString* private_key() const { return raw_ptr()->private_key_; }
-  RawArray* import_map() const { return raw_ptr()->import_map_; }
-  void set_import_map(const Array& map) const;
   bool LoadNotStarted() const {
     return raw_ptr()->load_state_ == RawLibrary::kAllocated;
   }
@@ -1742,7 +1740,6 @@
   void AddAnonymousClass(const Class& cls) const;
 
   // Library imports.
-  RawString* LookupImportMap(const String& name) const;
   void AddImport(const Library& library) const;
   RawLibrary* LookupImport(const String& url) const;