Revert "Switch libraries to using new tags."

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 c33eb63b7833a7e4f3f19d0651793f5cb043a559..eeb0c04fdef2e4f25040354dc318709b320b4728 100644
--- a/sdk/lib/core/errors.dart
+++ b/sdk/lib/core/errors.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of dart_core;
-
 class Error {
   const Error();
 
@@ -77,7 +75,7 @@ class ArgumentError implements Error {
 }
 
 /**
- * Error thrown because of an index outside of the valid range.
+ * Exception thrown because of an index outside of the valid range.
  *
  */
 class RangeError extends ArgumentError {