Make web DateTime.difference not assume platform implementation.

Changes `other._value` to `other.millisecondsSinceEpoch`,
which prevents crashing if provided with a different implementation
of `DateTime`.

Exposed by #58750.

Bug: https://dartbug.com/50750
Change-Id: I1dfc0bb4e8a44ef0df432c9a916a6620691e0547
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/276523
Reviewed-by: Stephen Adams <[email protected]>
Reviewed-by: Nate Bosch <[email protected]>
Commit-Queue: Stephen Adams <[email protected]>

Author: lrhn

sdk/lib/_internal/js_dev_runtime/patch/core_patch.dart

@@ -436,7 +436,7 @@ class DateTime {
 
   @patch
   Duration difference(DateTime other) {
-    return Duration(milliseconds: _value - other._value);
+    return Duration(milliseconds: _value - other.millisecondsSinceEpoch);
   }
 
   @patch

sdk/lib/_internal/js_runtime/lib/core_patch.dart

@@ -330,7 +330,7 @@ class DateTime {
 
   @patch
   Duration difference(DateTime other) {
-    return new Duration(milliseconds: _value - other._value);
+    return new Duration(milliseconds: _value - other.millisecondsSinceEpoch);
   }
 
   @patch