binding [nfc]: Add utcNow

This will be the same as `DateTime.timestamp()` in live code (therefore
the NFC). For testing, utcNow uses a clock instance that can be controlled
by FakeAsync.

We could have made call sites of `DateTime.now()` use it too, but
those for now don't need it for testing.

Author: PIG208

lib/model/binding.dart

@@ -120,6 +120,11 @@ abstract class ZulipBinding {
   /// This wraps [url_launcher.closeInAppWebView].
   Future<void> closeInAppWebView();
 
+  /// Provides access to the current UTC date and time.
+  ///
+  /// Outside tests, this just calls [DateTime.timestamp].
+  DateTime utcNow();
+
   /// Provides access to a new stopwatch.
   ///
   /// Outside tests, this just calls the [Stopwatch] constructor.
@@ -381,6 +386,9 @@ class LiveZulipBinding extends ZulipBinding {
     return url_launcher.closeInAppWebView();
   }
 
+  @override
+  DateTime utcNow() => DateTime.timestamp();
+
   @override
   Stopwatch stopwatch() => Stopwatch();
 

test/model/binding.dart

@@ -241,6 +241,9 @@ class TestZulipBinding extends ZulipBinding {
     _closeInAppWebViewCallCount++;
   }
 
+  @override
+  DateTime utcNow() => clock.now().toUtc();
+
   @override
   Stopwatch stopwatch() => clock.stopwatch();