Add Dart_PropagateError.

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
===================================================================
--- runtime/vm/isolate.h	(revision 3804)
+++ runtime/vm/isolate.h	(working copy)
@@ -26,7 +26,7 @@
 class ObjectPointerVisitor;
 class ObjectStore;
 class RawContext;
-class RawObject;
+class RawError;
 class StackResource;
 class StubCode;
 class Zone;
@@ -217,8 +217,8 @@
   MessageHandler* message_handler() const { return message_handler_; }
   void set_message_handler(MessageHandler* value) { message_handler_ = value; }
 
-  // Returns null on success, unhandled exception on failure.
-  RawObject* StandardRunLoop();
+  // Returns null on success, a RawError on failure.
+  RawError* StandardRunLoop();
 
   intptr_t ast_node_id() const { return ast_node_id_; }
   void set_ast_node_id(int value) { ast_node_id_ = value; }