Reapply conversion of most core interfaces to abstract classes.

lib/coreimpl/exceptions.dart

Index: lib/coreimpl/exceptions.dart
diff --git a/lib/coreimpl/exceptions.dart b/lib/coreimpl/exceptions.dart
index c2dd3a2f4df3754d168a340d568f1eee2b9ee756..72559ba6e231a91abf191d6fb948706c4db13b1b 100644
--- a/lib/coreimpl/exceptions.dart
+++ b/lib/coreimpl/exceptions.dart
@@ -3,7 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 class ExceptionImplementation implements Exception {
-  const ExceptionImplementation([msg = null]) : _msg = msg;
-  String toString() => (_msg === null) ? "Exception" : "Exception: $_msg";
-  final _msg;
+  final message;
+  const ExceptionImplementation([this.message]);
+  String toString() => (message == null) ? "Exception" : "Exception: $message";
 }