Start using Dart_PropagateError in the runtime/bin directory.

runtime/bin/builtin_natives.cc

Index: runtime/bin/builtin_natives.cc
===================================================================
--- runtime/bin/builtin_natives.cc	(revision 5191)
+++ runtime/bin/builtin_natives.cc	(working copy)
@@ -101,7 +101,11 @@
   const char* cstring = NULL;
   Dart_Handle result = Dart_StringToCString(str, &cstring);
   if (Dart_IsError(result)) {
-      cstring = Dart_GetError(result);
+    // TODO(turnidge): Consider propagating some errors here.  What if
+    // an isolate gets interrupted by the embedder in the middle of
+    // Dart_StringToCString?  We need to make sure not to swallow the
+    // interrupt.
+    cstring = Dart_GetError(result);
   }
   fprintf(out, "%s\n", cstring);
   fflush(out);