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

runtime/vm/parser.cc

Index: runtime/vm/parser.cc
===================================================================
--- runtime/vm/parser.cc	(revision 1469)
+++ runtime/vm/parser.cc	(working copy)
@@ -3100,7 +3100,7 @@
   Dart_Handle result = handler(tag,
                                Api::NewLocalHandle(library_),
                                Api::NewLocalHandle(url));
-  if (!Dart_IsValid(result)) {
+  if (Dart_IsError(result)) {
     ErrorMsg(token_pos, "library handler failed: %s", Dart_GetError(result));
   }
   return result;