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

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index 2ede7d1eda482b9aeddec89e43e1b5b06155c237..05ddfc4e3dddfd15175405f088c69e17456ca87b 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -64,6 +64,7 @@ DEFINE_FLAG(bool, throw_on_javascript_int_overflow, false,
 DEFINE_FLAG(bool, use_field_guards, true, "Guard field cids.");
 DEFINE_FLAG(bool, use_lib_cache, true, "Use library name cache");
 DEFINE_FLAG(bool, trace_field_guards, false, "Trace changes in field's cids.");
+DEFINE_FLAG(bool, use_mirrors, true, "Make dart:mirrors available.");
 
 DECLARE_FLAG(bool, error_on_bad_override);
 DECLARE_FLAG(bool, trace_compiler);
@@ -1188,20 +1189,25 @@ RawError* Object::Init(Isolate* isolate) {
   object_store->set_weak_property_class(cls);
   RegisterPrivateClass(cls, Symbols::_WeakProperty(), core_lib);
 
-  // Pre-register the mirrors library so we can place the vm class
-  // MirrorReference there rather than the core library.
-  lib = Library::LookupLibrary(Symbols::DartMirrors());
-  if (lib.IsNull()) {
-    lib = Library::NewLibraryHelper(Symbols::DartMirrors(), true);
-    lib.SetLoadRequested();
-    lib.Register();
-    object_store->set_bootstrap_library(ObjectStore::kMirrors, lib);
-  }
-  ASSERT(!lib.IsNull());
-  ASSERT(lib.raw() == Library::MirrorsLibrary());
+  if (FLAG_use_mirrors) {
+    // Pre-register the mirrors library so we can place the vm class
+    // MirrorReference there rather than the core library.
+    lib = Library::LookupLibrary(Symbols::DartMirrors());
+    if (lib.IsNull()) {
+      lib = Library::NewLibraryHelper(Symbols::DartMirrors(), true);
+      lib.SetLoadRequested();
+      lib.Register();
+      object_store->set_bootstrap_library(ObjectStore::kMirrors, lib);
+    }
+    ASSERT(!lib.IsNull());
+    ASSERT(lib.raw() == Library::MirrorsLibrary());
 
-  cls = Class::New<MirrorReference>();
-  RegisterPrivateClass(cls, Symbols::_MirrorReference(), lib);
+    cls = Class::New<MirrorReference>();
+    RegisterPrivateClass(cls, Symbols::_MirrorReference(), lib);
+  } else {
+    lib = Library::LookupLibrary(Symbols::DartMirrors());
+    ASSERT(lib.IsNull());
+  }
 
   // Pre-register the collection library so we can place the vm class
   // LinkedHashMap there rather than the core library.