Address additional comments on nullability

pkg/dev_compiler/tool/input_sdk/private/annotations.dart

Index: pkg/dev_compiler/tool/input_sdk/private/annotations.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/private/annotations.dart b/pkg/dev_compiler/tool/input_sdk/private/annotations.dart
index f900aec3112c5ce6ca223003002e9231014d84c8..3999f7c77ced7de8bd98fcd5cfb8bac0939e6c46 100644
--- a/pkg/dev_compiler/tool/input_sdk/private/annotations.dart
+++ b/pkg/dev_compiler/tool/input_sdk/private/annotations.dart
@@ -9,8 +9,11 @@ class ForceInline {
   const ForceInline();
 }
 
-/// Marks a variable or API to be non-nullable
-/// Currently unchecked
+/// Marks a variable or API to be non-nullable.
+/// ****CAUTION******
+/// This is currently unchecked, and hence should never be used
+/// on any public interface where user code could subclass, implement,
+/// or otherwise cause the contract to be violated.
 /// TODO(leafp): Consider adding static checking and exposing
 /// this to user code.
 class NotNull {
@@ -22,6 +25,9 @@ const notNull = const NotNull();
 /// Tells the development compiler to check a variable for null at its
 /// declaration point, and then to assume that the variable is non-null
 /// from that point forward.
+/// ****CAUTION******
+/// This is currently unchecked, and hence will not catch re-assignments
+/// of a variable with null
 class NullCheck {
   const NullCheck();
 }