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

runtime/vm/dart.cc

Index: runtime/vm/dart.cc
diff --git a/runtime/vm/dart.cc b/runtime/vm/dart.cc
index d14ba97bc2d846093787a9a39430bcb36b16ddbd..1f37ed9c1f1352eb19b636df29c8f78b8cff1a60 100644
--- a/runtime/vm/dart.cc
+++ b/runtime/vm/dart.cc
@@ -46,6 +46,7 @@ DEFINE_FLAG(bool, keep_code, false,
 
 DECLARE_FLAG(bool, print_class_table);
 DECLARE_FLAG(bool, trace_isolates);
+DECLARE_FLAG(bool, use_mirrors);
 
 Isolate* Dart::vm_isolate_ = NULL;
 ThreadPool* Dart::thread_pool_ = NULL;
@@ -277,6 +278,28 @@ RawError* Dart::InitializeIsolate(const uint8_t* snapshot_buffer, void* data) {
     if (!error.IsNull()) {
       return error.raw();
     }
+
+    if (!FLAG_use_mirrors) {
+      const Object& libmirrors =
+          Object::Handle(isolate->object_store()->mirrors_library());

[Ivan Posva, 2015/05/29 06:02:24]

Should we at this point set the value in the object store for the mirrors
library to null as well?

[rmacnak, 2015/06/13 00:11:09]

Actually, this is unsafe. We sometimes lookup libraries by id.

Updated to just be a check on imports. We'll really remove the library at build
time for precompilation.

+      if (!libmirrors.IsNull()) {
+        // Mirrors were enabled when the snapshot was taken. Retroactively
+        // remove them.
+        const GrowableObjectArray& old_libs =

[Ivan Posva, 2015/05/29 06:02:24]

Instead of creating a new array, you could find the index of the mirrors
library, overwrite it with the last entry and shrink they array.

+          GrowableObjectArray::Handle(isolate->object_store()->libraries());
+        const GrowableObjectArray& new_libs = GrowableObjectArray::Handle(
+            GrowableObjectArray::New(old_libs.Length(), Heap::kOld));
+        Object& lib = Object::Handle();
+        for (intptr_t i = 0; i < old_libs.Length(); i++) {
+          lib = old_libs.At(i);
+          if (lib.raw() != libmirrors.raw()) {
+            new_libs.Add(lib);
+          }
+        }
+        isolate->object_store()->set_libraries(new_libs);
+      }
+    }
+
     if (FLAG_trace_isolates) {
       isolate->heap()->PrintSizes();
       isolate->megamorphic_cache_table()->PrintSizes();