Reapply conversion of most core interfaces to abstract classes.

lib/coreimpl/date.dart

Index: lib/coreimpl/date.dart
diff --git a/lib/coreimpl/date.dart b/lib/coreimpl/date.dart
index 05a81fe9a3178b7520301b7e54bf56dbba12c4bf..82d9689cfa9596ed75e64aff79526d38f4413834 100644
--- a/lib/coreimpl/date.dart
+++ b/lib/coreimpl/date.dart
@@ -62,7 +62,7 @@ class DateImplementation implements Date {
   static const int _MAX_MILLISECONDS_SINCE_EPOCH = 8640000000000000;
 
   DateImplementation.fromMillisecondsSinceEpoch(this.millisecondsSinceEpoch,
-                                                [this.isUtc = false]) {
+                                                this.isUtc) {
     if (millisecondsSinceEpoch.abs() > _MAX_MILLISECONDS_SINCE_EPOCH) {
       throw new IllegalArgumentException(millisecondsSinceEpoch);
     }
@@ -159,6 +159,7 @@ class DateImplementation implements Date {
     return new Duration(milliseconds: ms - otherMs);
   }
 
+  // TODO(lrn): Make parameters not optional for the implementation class.
   external DateImplementation(int year,
                               [int month = 1,
                                int day = 1,