Introduce the Error object class in the vm. It represents all of the

runtime/vm/dart_api_impl.h

Index: runtime/vm/dart_api_impl.h
===================================================================
--- runtime/vm/dart_api_impl.h	(revision 3167)
+++ runtime/vm/dart_api_impl.h	(working copy)
@@ -102,11 +102,8 @@
   static Dart_Handle Success();
 
   // Generates a handle used to designate an error return.
-  static Dart_Handle Error(const char* format, ...);
+  static Dart_Handle NewError(const char* format, ...);
 
-  // Generates an error handle from an unhandled exception.
-  static Dart_Handle ErrorFromException(const Object& obj);
-
   // Gets a handle to Null.
   static Dart_Handle Null();