Move FutureImpl and CompleterImpl from coreimpl to core, while also making them private.

lib/core/future.dart

Index: lib/core/future.dart
diff --git a/lib/core/future.dart b/lib/core/future.dart
index 902432a64b90aa0a49be44957923cc0976fc79ea..80567c46f8ce0237a9ce2841acdfb07f39caae4f 100644
--- a/lib/core/future.dart
+++ b/lib/core/future.dart
@@ -29,7 +29,7 @@
  */
 abstract class Future<T> {
   /** A future whose value is immediately available. */
-  factory Future.immediate(T value) => new FutureImpl<T>.immediate(value);
+  factory Future.immediate(T value) => new _FutureImpl<T>.immediate(value);
 
   /** The value provided. Throws an exception if [hasValue] is false. */
   T get value;
@@ -161,7 +161,7 @@ abstract class Future<T> {
  */
 abstract class Completer<T> {
 
-  factory Completer() => new CompleterImpl<T>();
+  factory Completer() => new _CompleterImpl<T>();
 
   /** The future that will contain the value produced by this completer. */
   Future get future;