Add Dart_PropagateError.

runtime/vm/longjump.h

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_LONGJUMP_H_
 #define VM_LONGJUMP_H_
 
 #include <setjmp.h>
 
 #include "vm/allocation.h"
 
 namespace dart {
 
 class Error;
 
 class LongJump : public ValueObject {
  public:
   LongJump() : top_(NULL) { }
 
   jmp_buf* Set();
   void Jump(int value, const Error& error);
 
+  // Would it be safe to use this longjump?
+  //
+  // Checks to make sure that the jump would not cross Dart frames.
+  bool IsSafeToJump();
+
  private:
   jmp_buf environment_;
   StackResource* top_;
 
   DISALLOW_COPY_AND_ASSIGN(LongJump);
 };
 
 }  // namespace dart
 
 #endif  // VM_LONGJUMP_H_