Move DateImplementation from coreimpl to core, making it private.

runtime/lib/date_patch.dart

Index: runtime/lib/date_patch.dart
diff --git a/runtime/lib/date_patch.dart b/runtime/lib/date_patch.dart
index aee640f525ef17a0f4f377bee1dd0f8dc220f486..8e2b8cc04c21303fa0b347ca55067776b197e2cd 100644
--- a/runtime/lib/date_patch.dart
+++ b/runtime/lib/date_patch.dart
@@ -3,16 +3,16 @@
 // BSD-style license that can be found in the LICENSE file.
 // Dart core library.
 
-// VM implementation of DateImplementation.
-patch class DateImplementation {
-  /* patch */ DateImplementation(int year,
-                                 int month,
-                                 int day,
-                                 int hour,
-                                 int minute,
-                                 int second,
-                                 int millisecond,
-                                 bool isUtc)
+// VM implementation of _DateImpl.
+patch class _DateImpl {
+  /* patch */ _DateImpl(int year,
+                        int month,
+                        int day,
+                        int hour,
+                        int minute,
+                        int second,
+                        int millisecond,
+                        bool isUtc)
       : this.isUtc = isUtc,
         this.millisecondsSinceEpoch = _brokenDownDateToMillisecondsSinceEpoch(
             year, month, day, hour, minute, second, millisecond, isUtc) {
@@ -20,7 +20,7 @@ patch class DateImplementation {
     if (isUtc === null) throw new ArgumentError();
   }
 
-  /* patch */ DateImplementation.now()
+  /* patch */ _DateImpl.now()
       : isUtc = false,
         millisecondsSinceEpoch = _getCurrentMs() {
   }