Exception no longer an interface.

sdk/lib/core/exceptions.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.
 
 // Exceptions are thrown either by the VM or from Dart code.
 
 /**
  * Interface implemented by all core library exceptions.
  * Defaults to an implementation that only carries a simple message.
  */
-interface Exception default _ExceptionImplementation {
-  // TODO(lrn): This should be an abstract class, but we don't yet support
-  // redirecting factory constructors.
-  const Exception([var message]);
+abstract class Exception {
+  const factory Exception([var message]) = _ExceptionImplementation;
 }
 
 
 /** Default implementation of [Exception] which carries a message. */
 class _ExceptionImplementation implements Exception {
   final message;
   const _ExceptionImplementation([this.message]);
   String toString() => (message == null) ? "Exception" : "Exception: $message";
 }
 
@@ skipping 50 matching lines @@
 }
 
 /**
  * Exception thrown when a runtime error occurs.
  */
 class RuntimeError implements Exception {
   final message;
   RuntimeError(this.message);
   String toString() => "RuntimeError: $message";
 }