Add Dart_Null, Dart_True, and Dart_False.

runtime/vm/dart_api_state.h

Index: runtime/vm/dart_api_state.h
===================================================================
--- runtime/vm/dart_api_state.h	(revision 1035)
+++ runtime/vm/dart_api_state.h	(working copy)
@@ -72,6 +72,11 @@
 };
 
 
+// A distinguished callback which indicates that a persistent handle
+// should not be deleted from the dart api.
+void ProtectedHandleCallback();
+
+
 // Implementation of persistent handles which are handed out through the
 // dart API.
 class PersistentHandle {
@@ -91,6 +96,11 @@
   intptr_t type() const { return type_; }
   void set_type(intptr_t value) { type_ = value; }
 
+  // Some handles are protected from being freed via the external dart api.
+  bool IsProtected() {
+    return callback() == &ProtectedHandleCallback;
+  }
+
  private:
   friend class PersistentHandles;
 
@@ -257,17 +267,25 @@
 // basis and destroyed when the isolate is shutdown.
 class ApiState {
  public:
-  ApiState() : top_scope_(NULL), true_(NULL) { }
+  ApiState() : top_scope_(NULL), null_(NULL), true_(NULL), false_(NULL) { }
   ~ApiState() {
     while (top_scope_ != NULL) {
       ApiLocalScope* scope = top_scope_;
       top_scope_ = top_scope_->previous();
       delete scope;
     }
+    if (null_ != NULL) {
+      persistent_handles().FreeHandle(null_);
+      null_ = NULL;
+    }
     if (true_ != NULL) {
       persistent_handles().FreeHandle(true_);
       true_ = NULL;
     }
+    if (false_ != NULL) {
+      persistent_handles().FreeHandle(false_);
+      false_ = NULL;
+    }
   }
 
   // Accessors.
@@ -327,6 +345,18 @@
     }
     return total;
   }
+  PersistentHandle* Null() {
+    if (null_ == NULL) {
+      Zone zone;  // Setup a VM zone as we are creating some handles.
+      HandleScope scope;  // Setup a VM handle scope.
+
+      Object& null_object = Object::Handle();
+      null_ = persistent_handles().AllocateHandle();
+      null_->set_raw(null_object);
+      null_->set_callback(reinterpret_cast<void*>(&ProtectedHandleCallback));
+    }
+    return null_;
+  }
   PersistentHandle* True() {
     if (true_ == NULL) {
       Zone zone;  // Setup a VM zone as we are creating some handles.
@@ -335,16 +365,31 @@
       const Object& true_object = Object::Handle(Bool::True());
       true_ = persistent_handles().AllocateHandle();
       true_->set_raw(true_object);
+      true_->set_callback(reinterpret_cast<void*>(&ProtectedHandleCallback));
     }
     return true_;
   }
+  PersistentHandle* False() {
+    if (false_ == NULL) {
+      Zone zone;  // Setup a VM zone as we are creating some handles.
+      HandleScope scope;  // Setup a VM handle scope.
 
+      const Object& false_object = Object::Handle(Bool::False());
+      false_ = persistent_handles().AllocateHandle();
+      false_->set_raw(false_object);
+      false_->set_callback(reinterpret_cast<void*>(&ProtectedHandleCallback));
+    }
+    return false_;
+  }
+
  private:
   PersistentHandles persistent_handles_;
   ApiLocalScope* top_scope_;
 
-  // A persistent handle to the "True" object.
+  // Persistent handles to important objects.
+  PersistentHandle* null_;
   PersistentHandle* true_;
+  PersistentHandle* false_;
 
   DISALLOW_COPY_AND_ASSIGN(ApiState);
 };