Enforce rule in vm that factory name must match enclosing class name.

runtime/lib/error.dart

Index: runtime/lib/error.dart
===================================================================
--- runtime/lib/error.dart	(revision 15427)
+++ runtime/lib/error.dart	(working copy)
@@ -53,7 +53,7 @@
 class CastErrorImplementation
     extends TypeErrorImplementation
     implements CastError {
-  factory CastError._uninstantiable() {
+  factory CastErrorImplementation._uninstantiable() {
     throw new UnsupportedError(
         "CastError can only be allocated by the VM");
   }