Add Dart_PropagateError.

runtime/vm/exceptions.h

Index: runtime/vm/exceptions.h
===================================================================
--- runtime/vm/exceptions.h	(revision 3804)
+++ runtime/vm/exceptions.h	(working copy)
@@ -11,14 +11,16 @@
 namespace dart {
 
 // Forward declarations.
+class Error;
 class Instance;
 class Object;
-class RawInstance;
+class RawObject;
 
 class Exceptions : AllStatic {
  public:
   static void Throw(const Instance& exception);
   static void ReThrow(const Instance& exception, const Instance& stacktrace);
+  static void PropagateError(const Object& obj);
 
   enum ExceptionType {
     kIndexOutOfRange,
@@ -37,8 +39,10 @@
 
   static void ThrowByType(ExceptionType type,
                           const GrowableArray<const Object*>& arguments);
-  static RawInstance* Create(ExceptionType type,
-                             const GrowableArray<const Object*>& arguments);
+  // Returns a RawInstance if the exception is successfully created,
+  // otherwise returns a RawError.
+  static RawObject* Create(ExceptionType type,
+                           const GrowableArray<const Object*>& arguments);
 
  private:
   DISALLOW_COPY_AND_ASSIGN(Exceptions);