Remove some exceptions from mock.dart.

pkg/dartdoc/dartdoc.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.
 
 /**
  * To generate docs for a library, run this script with the path to an
  * entrypoint .dart file, like:
  *
  *     $ dart dartdoc.dart foo.dart
  *
@@ skipping 1709 matching lines @@
   }
 
   /**
    * Returns [:true:] if [type] should be regarded as an exception.
    */
   bool isException(TypeMirror type) {
     return type.simpleName.endsWith('Exception');
   }
 }
 
+/**
+ * Used to report an unexpected error in the DartDoc tool or the
+ * underlying data
+ */
+class InternalError {
+  final String message;
+  const InternalError(this.message);
+  String toString() => "InternalError: '$message'";
+}