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 5fbd2d99df880cd324ea1d50c0d3452f8fac8f1a..c18e174e0190334025f5a67b5a23fd1127245d58 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 {