[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: https://github.com/dart-lang/sdk/issues/50821
Change-Id: I1dfc0bb4e8a44ef0df432c9a916a6620691e0547
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/277002
Commit-Queue: Lasse Nielsen <lrn@google.com>
Reviewed-by: Nate Bosch <nbosch@google.com>
Auto-Submit: Lasse Nielsen <lrn@google.com>

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

diff --git a/sdk/lib/_internal/js_dev_runtime/patch/core_patch.dart b/sdk/lib/_internal/js_dev_runtime/patch/core_patch.dart
index 567e9f1..50ee91c 100644
--- a/sdk/lib/_internal/js_dev_runtime/patch/core_patch.dart
+++ b/sdk/lib/_internal/js_dev_runtime/patch/core_patch.dart
@@ -451,7 +451,7 @@
 
   @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

diff --git a/sdk/lib/_internal/js_runtime/lib/core_patch.dart b/sdk/lib/_internal/js_runtime/lib/core_patch.dart
index 396ab47..603aad8a 100644
--- a/sdk/lib/_internal/js_runtime/lib/core_patch.dart
+++ b/sdk/lib/_internal/js_runtime/lib/core_patch.dart
@@ -345,7 +345,7 @@
 
   @patch
   Duration difference(DateTime other) {
-    return new Duration(milliseconds: _value - other._value);
+    return new Duration(milliseconds: _value - other.millisecondsSinceEpoch);
   }
 
   @patch