Make Exception a class, not an interface, and remove the const constructor.

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..03c0d4178a94345c1255caf8eec0cca79ed045b8 100644
--- a/sdk/lib/core/exceptions.dart
+++ b/sdk/lib/core/exceptions.dart
@@ -5,13 +5,11 @@
 // Exceptions are thrown either by the VM or from Dart code.
 
 /**
- * Interface implemented by all core library exceptions.
+ * Marker 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  {
+  factory Exception([var message]) => new _ExceptionImplementation(message);
 }