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

runtime/vm/bootstrap_natives.cc

Index: runtime/vm/bootstrap_natives.cc
diff --git a/runtime/vm/bootstrap_natives.cc b/runtime/vm/bootstrap_natives.cc
index 07c492804792a93bb1513837506a617a723ee6fc..e21045fd317e09b31c629fce11a05993e39ccdd2 100644
--- a/runtime/vm/bootstrap_natives.cc
+++ b/runtime/vm/bootstrap_natives.cc
@@ -116,9 +116,10 @@ void Bootstrap::SetupNativeResolver() {
   library.set_native_entry_symbol_resolver(symbol_resolver);
 
   library = Library::MirrorsLibrary();
-  ASSERT(!library.IsNull());
-  library.set_native_entry_resolver(resolver);
-  library.set_native_entry_symbol_resolver(symbol_resolver);
+  if (!library.IsNull()) {
+    library.set_native_entry_resolver(resolver);
+    library.set_native_entry_symbol_resolver(symbol_resolver);
+  }
 
   library = Library::ProfilerLibrary();
   ASSERT(!library.IsNull());