[beta] 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 #50750.

Bug: https://dartbug.com/50750
Cherry-pick: #50821
Change-Id: I1dfc0bb4e8a44ef0df432c9a916a6620691e0547
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/277002
Commit-Queue: Lasse Nielsen <[email protected]>
Reviewed-by: Nate Bosch <[email protected]>
Auto-Submit: Lasse Nielsen <[email protected]>

Author: lrhn

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

@@ -451,7 +451,7 @@ 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

@@ -345,7 +345,7 @@ class DateTime {
 
   @patch
   Duration difference(DateTime other) {
-    return new Duration(milliseconds: _value - other._value);
+    return new Duration(milliseconds: _value - other.millisecondsSinceEpoch);
   }
 
   @patch