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 7ce86289ebc5f80a42b5e5def06444527435701a..8963feb81e69395b268afdc48d61378b167434d8 100644
--- a/sdk/lib/core/errors.dart
+++ b/sdk/lib/core/errors.dart
@@ -25,6 +25,14 @@ 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 {