Expose remaining 'errors' in an unified dart:core.

corelib/src/errors.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // 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.
 
 class AssertionError {
 }
+
+class TypeError extends AssertionError {
+}
+
+class CastException extends TypeError {
+}
+
+class FallThroughError {
+  const FallThroughError();
+}