[kernel] Fix assert message.

runtime/lib/errors_patch.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.
 
 @patch
 class Error {
   @patch
   static String _objectToString(Object object) {
     return Object._toString(object);
   }
@@ skipping 37 matching lines @@
   }
 
   String get _messageString {
     if (message == null) return "is not true.";
     if (message is String) return message;
     return Error.safeToString(message);
   }
 
   String toString() {
     if (_url == null) {
-      if (message == null) return _failedAssertion;
-      return "'$_failedAssertion': $_messageString";
+      if (message == null) return _failedAssertion?.trim();
+      return "'${_failedAssertion?.trim()}': $_messageString";
     }
     var columnInfo = "";
     if (_column > 0) {
       // Only add column information if it is valid.
       columnInfo = " pos $_column";
     }
     return "'$_url': Failed assertion: line $_line$columnInfo: "
         "'$_failedAssertion': $_messageString";
   }
 
@@ skipping 359 matching lines @@
   _DuplicatedFieldInitializerError(this._name);
 
   toString() => "Error: field '$_name' is already initialized.";
 }
 
 @patch
 class _ConstantExpressionError {
   @patch
   _throw(error) => throw error;
 }