Fix remaining instance where DateTime accesses the private member _value

This does not work if the DateTime accessed is an implementation of
the DateTime interface, rather than an instance of the same class.

Related issues:

* #34962
* srawlins/timezone#57

Related changes:

* https://dart-review.googlesource.com/c/sdk/+/81828

Closes #42351
#42351

GitOrigin-RevId: 877ae83
Change-Id: I91205292bb91d7958c4b3687135a8152d6ebbe6e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/151322
Reviewed-by: Siva Annamalai <[email protected]>
Commit-Queue: Michael Thomsen <[email protected]>

sdk/lib/_internal/vm/lib/date_patch.dart

@@ -198,7 +198,7 @@ class DateTime {
 
   @patch
   Duration difference(DateTime other) {
-    return new Duration(microseconds: _value - other._value);
+    return new Duration(microseconds: _value - other.microsecondsSinceEpoch);
   }
 
   @patch