Updates the corelib to the new default specification for interfaces and new factory syntax

corelib/src/duration.dart

Index: corelib/src/duration.dart
diff --git a/corelib/src/duration.dart b/corelib/src/duration.dart
index da52a274ddd9f87c03a82aaac219c71a5677612d..f492f896dd96d740c6450e0d7a7cb2037aaa7f48 100644
--- a/corelib/src/duration.dart
+++ b/corelib/src/duration.dart
@@ -7,7 +7,7 @@
 /**
  * A [Duration] represents a time span. A duration can be negative.
  */
-interface Duration extends Comparable factory DurationImplementation {
+interface Duration extends Comparable default DurationImplementation {
   /**
    * The duration is the sum of all individual parts. This means that individual
    * parts don't need to be less than the next-bigger unit. For example [hours]