Add DateTime.timestamp() constructor for "now as UTC".

Change-Id: I497f335b1f1eb0f691c2d44557c50f0f55358426
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/292003
Reviewed-by: Slava Egorov <vegorov@google.com>
Reviewed-by: Aske Simon Christensen <askesc@google.com>
Auto-Submit: Lasse Nielsen <lrn@google.com>
Reviewed-by: Nate Bosch <nbosch@google.com>
Commit-Queue: Lasse Nielsen <lrn@google.com>
Reviewed-by: Stephen Adams <sra@google.com>

CHANGELOG.md

@@ -30,6 +30,7 @@
 
 #### `dart:core`
 - Added `bool.parse` and `bool.tryParse` static methods.
+- Added `DateTime.timestamp()` constructor to get current time as UTC.
 
 - **Breaking change** [#49529][]:
   - Removed the deprecated `List` constructor, as it wasn't null safe.

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

@@ -373,6 +373,11 @@ class DateTime {
       : isUtc = false,
         _value = Primitives.dateNow();
 
+  @patch
+  DateTime._nowUtc()
+      : isUtc = true,
+        _value = Primitives.dateNow();
+
   /// Rounds the given [microsecond] to the nearest milliseconds value.
   ///
   /// For example, invoked with argument `2600` returns `3`.

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

@@ -280,6 +280,11 @@ class DateTime {
       : isUtc = false,
         _value = Primitives.dateNow();
 
+  @patch
+  DateTime._nowUtc()
+      : isUtc = true,
+        _value = Primitives.dateNow();
+
   /// Rounds the given [microsecond] to the nearest milliseconds value.
   ///
   /// For example, invoked with argument `2600` returns `3`.

sdk/lib/_internal/vm_shared/lib/date_patch.dart

@@ -66,6 +66,11 @@ class DateTime {
       : isUtc = false,
         _value = _getCurrentMicros();
 
+  @patch
+  DateTime._nowUtc()
+      : isUtc = true,
+        _value = _getCurrentMicros();
+
   @patch
   String get timeZoneName {
     if (isUtc) return "UTC";

sdk/lib/core/date_time.dart

@@ -225,6 +225,14 @@ class DateTime implements Comparable<DateTime> {
   /// ```
   DateTime.now() : this._now();
 
+  /// Constructs a [DateTime] with the current UTC date and time.
+  ///
+  ///
+  /// ```dart
+  /// final mark = DateTime.timestamp();
+  /// ```
+  DateTime.timestamp() : this._nowUtc();
+
   /// Constructs a new [DateTime] instance based on [formattedString].
   ///
   /// Throws a [FormatException] if the input string cannot be parsed.

tests/corelib/date_time_test.dart

@@ -30,6 +30,45 @@ void testNow() {
   Expect.isFalse(t1.isUtc);
 }
 
+void testTimestamp() {
+  // Assume `DateTime.now` works.
+  const int N = 1000000;
+  var nowBefore = DateTime.now();
+  var timestamp = DateTime.timestamp();
+  var iterations = 0;
+  while (true) {
+    // Test that `timestamp` is using the same "current time" as `now`.
+    Expect.isTrue(timestamp.isUtc, "Is UTC");
+    Expect.isTrue(
+        nowBefore.microsecondsSinceEpoch <= timestamp.microsecondsSinceEpoch,
+        "After an earlier now");
+
+    var laterNow = DateTime.now();
+    Expect.isTrue(
+        timestamp.microsecondsSinceEpoch <= laterNow.microsecondsSinceEpoch,
+        "Before a later now");
+
+    var newTimestamp = DateTime.timestamp();
+
+    // Succeed if time has also progressed.
+    if (timestamp.microsecondsSinceEpoch <
+        newTimestamp.microsecondsSinceEpoch) {
+      break;
+    }
+    // Otherwise emit diagnostics occasionally, until the test times out
+    // or succeeds. Which should be in at most a few milliseconds if all
+    // is well.
+    if (++iterations >= N) {
+      print("testTimestamp: No DateTime.timestamp() progress in $N loops."
+          "Time: $timestamp");
+      iterations = 0;
+    }
+
+    nowBefore = laterNow;
+    timestamp = newTimestamp;
+  }
+}
+
 void testMillisecondsSinceEpoch() {
   var dt1 = new DateTime.now();
   var millisecondsSinceEpoch = dt1.millisecondsSinceEpoch;
@@ -1218,6 +1257,7 @@ void testRegression46966() {
 
 void main() {
   testNow();
+  testTimestamp();
   testMillisecondsSinceEpoch();
   testMicrosecondsSinceEpoch();
   testConstructors();