Revert "Remove NullPointerException."

sdk/lib/core/errors.dart

Index: sdk/lib/core/errors.dart
diff --git a/sdk/lib/core/errors.dart b/sdk/lib/core/errors.dart
index 8fbfe16c09f4ba5cc3414599342339283c99a8a9..7ce86289ebc5f80a42b5e5def06444527435701a 100644
--- a/sdk/lib/core/errors.dart
+++ b/sdk/lib/core/errors.dart
@@ -25,21 +25,13 @@ class CastError implements Error {
 }
 
 /**
- * Error thrown when attempting to throw [:null:].
- */
-class NullThrownError implements Error {
-  const NullThrownError();
-  String toString() => "Throw of null.";
-}
-
-/**
  * Error thrown when a function is passed an unacceptable argument.
  */
 class ArgumentError implements Error {
   final message;
 
   /** The [message] describes the erroneous argument. */
-  ArgumentError([this.message]);
+  const ArgumentError([this.message]);
 
   String toString() {
     if (message != null) {