Add optional message argument to assert statements in the VM.

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.
 
 part of dart.core;
 
 /**
  * Error objects thrown in the case of a program failure.
  *
  * An `Error` object represents a program failure that the programmer
@@ skipping 78 matching lines @@
 
   external static String _objectToString(Object object);
 
   external StackTrace get stackTrace;
 }
 
 /**
  * Error thrown by the runtime system when an assert statement fails.
  */
 class AssertionError extends Error {
+  /** Message describing the assertion error. */
+  final Object message;

[sra1, 2015/09/02 18:00:27]

1. Is the intention to also give TypeError a message field?

2. This is a breaking change for anyone who implements AssertionError or
TypeError as they have to now implement get:message.  I'm not bothered by that
but I would like some clarity on what aTypeError.message is expected to return. 
The same as toString()?

[Lasse Reichstein Nielsen, 2015/09/03 07:42:04]

No, but it's an inadvertent consequence of making AssertionError have a field
and TypeError being defined as subclassing AssertionError.
I.e., I'm aware of it, and didn't find a way to avoid it (short of not making
TypeError an AssertionError).
For the record, it is *not* a breaking change.
No existing code will stop working.

It will make some libraries be outdated and cause warnings,
and if you start using the "message" field on objects from those libraries, your
code will not work, but that won't happen by itself.
Probably null since you don't have a way to specify a message for type
assertions.
Which suggests that making the message public might not be such a great idea
anyway. That, or making TypeError an AssertionError was the mistake.

+  AssertionError([this.message]);
 }
 
 /**
  * Error thrown by the runtime system when a type assertion fails.
  */
 class TypeError extends AssertionError {
 }
 
 /**
  * Error thrown by the runtime system when a cast operation fails.
@@ skipping 453 matching lines @@
  * the first time it is read. If evaluating the initializer expression causes
  * another read of the variable, this error is thrown.
  */
 class CyclicInitializationError extends Error {
   final String variableName;
   CyclicInitializationError([this.variableName]);
   String toString() => variableName == null
       ? "Reading static variable during its initialization"
       : "Reading static variable '$variableName' during its initialization";
 }