Reapply "Exception no longer an interface."

sdk/lib/core/exceptions.dart

Index: sdk/lib/core/exceptions.dart
diff --git a/sdk/lib/core/exceptions.dart b/sdk/lib/core/exceptions.dart
index 2d2bc436dd496be9903aa7ac5529310df31ee8d4..61f3f89def79b09ac9016b15ec8100704051971a 100644
--- a/sdk/lib/core/exceptions.dart
+++ b/sdk/lib/core/exceptions.dart
@@ -8,10 +8,8 @@
  * Interface implemented by all core library exceptions.
  * Defaults to an implementation that only carries a simple message.
  */
-interface Exception default _ExceptionImplementation {
-  // TODO(lrn): This should be an abstract class, but we don't yet support
-  // redirecting factory constructors.
-  const Exception([var message]);
+abstract class Exception {
+  const factory Exception([var message]) = _ExceptionImplementation;
 }