Add a VM defined class MirrorReference as an opaque pointer for Dart code to VM internal objects.

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
===================================================================
--- runtime/vm/raw_object.h	(revision 24693)
+++ runtime/vm/raw_object.h	(working copy)
@@ -66,6 +66,7 @@
     V(Stacktrace)                                                              \
     V(JSRegExp)                                                                \
     V(WeakProperty)                                                            \
+    V(MirrorReference)                                                         \
     V(DartFunction)                                                            \
     V(Float32x4)                                                               \
     V(Uint32x4)                                                                \
@@ -1526,7 +1527,21 @@
   friend class ScavengerVisitor;
 };
 
+// MirrorReferences are used by mirrors to hold reflectees that are VM
+// internal objects, such as libraries, classes, functions or types.
+class RawMirrorReference : public RawInstance {
+  RAW_HEAP_OBJECT_IMPLEMENTATION(MirrorReference);
 
+  RawObject** from() {
+    return reinterpret_cast<RawObject**>(&ptr()->referent_);
+  }
+  RawObject* referent_;
+  RawObject** to() {
+    return reinterpret_cast<RawObject**>(&ptr()->referent_);
+  }
+};
+
+
 // Class Id predicates.
 
 inline bool RawObject::IsErrorClassId(intptr_t index) {