add a named constructor to FrameTiming

lib/ui/window.dart

@@ -92,6 +92,28 @@ class FrameTiming {
   FrameTiming(List<int> timestamps)
       : assert(timestamps.length == FramePhase.values.length), _timestamps = timestamps;
 
+  /// Construct [FrameTiming] with given timestamp in micrseconds.
+  ///
+  /// This constructor is used for unit test only. Real [FrameTiming]s should
+  /// be retrieved from [Window.onReportTimings].
+  ///
+  /// TODO(CareF): This is part of #20229. Remove back to default constructor
+  /// after #20229 lands and corresponding framwork PRs land.
+  factory FrameTiming.fromTimeStamps({
+    int? vsyncStart,
+    required int buildStart,
+    required int buildFinish,
+    required int rasterStart,
+    required int rasterFinish
+  }) {
+    return FrameTiming(<int>[
+      buildStart,
+      buildFinish,
+      rasterStart,
+      rasterFinish
+    ]);
+  }
+
   /// This is a raw timestamp in microseconds from some epoch. The epoch in all
   /// [FrameTiming] is the same, but it may not match [DateTime]'s epoch.
   int timestampInMicroseconds(FramePhase phase) => _timestamps[phase.index];

lib/web_ui/lib/src/ui/window.dart

@@ -1033,6 +1033,21 @@ class FrameTiming {
       : assert(timestamps.length == FramePhase.values.length),
         _timestamps = timestamps;
 
+  /// Construct [FrameTiming] with given timestamp in micrseconds.
+  ///
+  /// This constructor is used for unit test only. Real [FrameTiming]s should
+  /// be retrieved from [Window.onReportTimings].
+  ///
+  /// TODO(CareF): This is part of #20229. Remove back to default constructor
+  /// after #20229 lands and corresponding framwork PRs land.
+  FrameTiming.fromTimeStamps({
+    int? vsyncStart,
+    required int buildStart,
+    required int buildFinish,
+    required int rasterStart,
+    required int rasterFinish
+  }) : _timestamps = <int>[buildStart, buildFinish, rasterStart, rasterFinish];
+
   /// This is a raw timestamp in microseconds from some epoch. The epoch in all
   /// [FrameTiming] is the same, but it may not match [DateTime]'s epoch.
   int timestampInMicroseconds(FramePhase phase) => _timestamps[phase.index];