Deal with unhandled exceptions the same way in all Dart api functions.

runtime/vm/dart_api_impl.h

Index: runtime/vm/dart_api_impl.h
===================================================================
--- runtime/vm/dart_api_impl.h	(revision 1328)
+++ runtime/vm/dart_api_impl.h	(working copy)
@@ -14,6 +14,7 @@
 class Object;
 class PersistentHandle;
 class RawObject;
+class UnhandledException;
 
 class Api : AllStatic {
  public:
@@ -39,6 +40,9 @@
 
   static Dart_Handle VError(const char* format, va_list args);
 
+  // Generates an error handle from an unhandled exception.
+  static Dart_Handle ErrorFromException(const Object& obj);
+
   // Gets a handle to Null.
   static Dart_Handle Null();