Implement spawnFunction from the new isolate api.

runtime/vm/dart_entry.h

Index: runtime/vm/dart_entry.h
===================================================================
--- runtime/vm/dart_entry.h	(revision 5367)
+++ runtime/vm/dart_entry.h	(working copy)
@@ -17,6 +17,7 @@
 class Function;
 class Instance;
 class Integer;
+class Library;
 class Object;
 class RawInstance;
 class RawObject;
@@ -62,6 +63,7 @@
  public:
   // On success, returns a RawInstance.  On failure, a RawError.
   static RawObject* ExceptionCreate(
+      const Library& library,
       const String& exception_name,
       const GrowableArray<const Object*>& arguments);