Reapply conversion of most core interfaces to abstract classes.

lib/core/exceptions.dart

Index: lib/core/exceptions.dart
diff --git a/lib/core/exceptions.dart b/lib/core/exceptions.dart
index b6ab8257be2227227af2c292209334c14c7cbc3d..01020018cae5b718051a915ef6a213e0ec0d811e 100644
--- a/lib/core/exceptions.dart
+++ b/lib/core/exceptions.dart
@@ -6,9 +6,10 @@
 
 /**
  * Interface implemented by all core library exceptions.
+ * Defaults to an implementation that only carries a simple message.
  */
 interface Exception default ExceptionImplementation {
-  const Exception([var msg]);
+  const Exception([var message]);
 }