Make RuntimeError an Error

sdk/lib/core/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 Error {
   const Error();
 
   /**
    * Safely convert a value to a [String] description.
    *
@@ skipping 227 matching lines @@
 
 class OutOfMemoryError implements Error {
   const OutOfMemoryError();
   String toString() => "Out of Memory";
 }
 
 class StackOverflowError implements Error {
   const StackOverflowError();
   String toString() => "Stack Overflow";
 }
+
+/**
+ * Error thrown when a runtime error occurs.
+ */
+class RuntimeError implements Error {
+  final message;
+  RuntimeError(this.message);
+  String toString() => "RuntimeError: $message";
+}