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

runtime/vm/dart_api_impl.cc

Index: runtime/vm/dart_api_impl.cc
diff --git a/runtime/vm/dart_api_impl.cc b/runtime/vm/dart_api_impl.cc
index d56cb0f5b8f25948cdb9a5fc6ab1244c0a2fd909..edb48437a1fa532597666825c08daa6096d31c55 100644
--- a/runtime/vm/dart_api_impl.cc
+++ b/runtime/vm/dart_api_impl.cc
@@ -5466,10 +5466,12 @@ DART_EXPORT Dart_Handle Dart_FinalizeLoading(bool complete_futures) {
   // Notify mirrors that MirrorSystem.libraries needs to be recomputed.
   const Library& libmirrors =
       Library::Handle(isolate, Library::MirrorsLibrary());
-  const Field& dirty_bit = Field::Handle(isolate,
-      libmirrors.LookupLocalField(String::Handle(String::New("dirty"))));
-  ASSERT(!dirty_bit.IsNull() && dirty_bit.is_static());
-  dirty_bit.set_value(Bool::True());
+  if (!libmirrors.IsNull()) {

[Ivan Posva, 2015/05/29 06:02:25]

Here we are not relying on the flag being set, but on the fact that the mirrors
library is null instead of the flag value, which makes me think that we should
reset it as necessary.

+    const Field& dirty_bit = Field::Handle(isolate,
+        libmirrors.LookupLocalField(String::Handle(String::New("dirty"))));
+    ASSERT(!dirty_bit.IsNull() && dirty_bit.is_static());
+    dirty_bit.set_value(Bool::True());
+  }
 
   if (complete_futures) {
     const Library& corelib = Library::Handle(isolate, Library::CoreLibrary());