Add a runtime flag to control availability of dart:mirrors.

runtime/bin/dartutils.cc

Index: runtime/bin/dartutils.cc
diff --git a/runtime/bin/dartutils.cc b/runtime/bin/dartutils.cc
index c443fc91f6ed86d008b3a41397230ab16a79b657..afe5a7e22f890e748dadc40f72ac12934267995b 100644
--- a/runtime/bin/dartutils.cc
+++ b/runtime/bin/dartutils.cc
@@ -392,8 +392,8 @@ Dart_Handle DartUtils::LibraryTagHandler(Dart_LibraryTag tag,
       if (DartUtils::IsDartIOLibURL(url_string)) {
         return Builtin::LoadLibrary(url, Builtin::kIOLibrary);
       }
-      return NewError("The built-in library '%s' is not available"
-                      " on the stand-alone VM.\n", url_string);
+      return NewError("The built-in library '%s' is not available\n",
+                      url_string);
     } else {
       ASSERT(tag == Dart_kSourceTag);
       return NewError("Unable to load source '%s' ", url_string);