Make compile-time errors catchable

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 553 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";
 }
+
+
+class SyntaxError extends Error {

[Lasse Reichstein Nielsen, 2016/09/15 05:41:35]

I have two objections here:

1. This will represent compile-time errors, not all of which are syntax errors.
A better name would be `CompileTimeError`.

2. I'm not sure I want to expose this class. The dart2js code won't need it for
anything, and its current implementation isn't adding anything important for the
user - it's just wrapping a message.
I hope no user will want to catch a CompileTimeError directly. If you catch all
errors, it makes sense to catch CompileTimeError too, but singling out
CompileTimeErrors with an `on CompileTimeError catch (e)` is probably just being
too clever.
Making it a VM-private class that still extends Error is sufficient for me, and
I'd like to see if it's sufficient for other users as well.

The subclasses of Error that we have are there because they carry important
information for debugging and for creating a good error string in a consistent
way, and not really so you can identify them by class (not that people don't but
it's not the intent). This class does neither: It doesn't expose important
information as data, and it isn't intended for users to create instances of to
help them have consistent error messages, so it has no reason to be user
visible.

[hausner, 2016/09/15 21:28:28]

Renamed SyntaxError to _CompileTimeError. The class is VM-only and not
user-visible.

+  final String errorMsg;
+  SyntaxError(this.errorMsg);
+  String toString() => errorMsg;
+}
+