[flutter_test] Fix incorrect missed budget count (#95003)

packages/flutter_test/lib/src/frame_timing_summarizer.dart

@@ -294,16 +294,21 @@ class FrameTimingSummarizer {
       };
 }
 
-// The following helper functions require data sorted
-
-// return the 100*p-th percentile of the data
+/// Returns the 100*p-th percentile of [data].
+///
+/// [data] must be sorted in ascending order.
 T _findPercentile<T>(List<T> data, double p) {
   assert(p >= 0 && p <= 1);
   return data[((data.length - 1) * p).round()];
 }
 
-// return the number of items in data that > threshold
+/// Returns the number of elements in [data] that exceed [threshold].
+///
+/// [data] must be sorted in ascending order.
 int _countExceed<T extends Comparable<T>>(List<T> data, T threshold) {
-  return data.length -
-      data.indexWhere((T datum) => datum.compareTo(threshold) > 0);
+  final int exceedsThresholdIndex = data.indexWhere((T datum) => datum.compareTo(threshold) > 0);
+  if (exceedsThresholdIndex == -1) {
+    return 0;
+  }
+  return data.length - exceedsThresholdIndex;
 }

packages/flutter_test/test/frame_timing_summarizer_test.dart

@@ -7,7 +7,8 @@ import 'dart:ui';
 import 'package:flutter_test/flutter_test.dart';
 
 void main() {
-  test('Test FrameTimingSummarizer', () {
+  group(FrameTimingSummarizer, () {
+    test('calculates all fields', () {
       List<int> vsyncTimes = <int>[
         for (int i = 0; i < 100; i += 1) 100 * (i + 1),
       ];
@@ -54,4 +55,87 @@ void main() {
       expect(summary.p99VsyncOverhead.inMicroseconds, 9900);
       expect(summary.worstVsyncOverhead.inMicroseconds, 10000);
     });
+
+    group('missed budget count', () {
+      test('when single element missed budget', () {
+        final FrameTimingSummarizer summary = FrameTimingSummarizer(<FrameTiming>[
+          FrameTiming(
+            buildStart: 0,
+            buildFinish: (kBuildBudget + const Duration(microseconds: 1)).inMicroseconds,
+            vsyncStart: 0,
+            rasterStart: 0,
+            rasterFinish: 0,
+            rasterFinishWallTime: 0,
+          ),
+        ]);
+        expect(summary.missedFrameBuildBudget, 1);
+      });
+
+      test('when single element within budget', () {
+        final FrameTimingSummarizer summary = FrameTimingSummarizer(<FrameTiming>[
+          FrameTiming(
+            buildStart: 0,
+            buildFinish: 0,
+            vsyncStart: 0,
+            rasterStart: 0,
+            rasterFinish: 0,
+            rasterFinishWallTime: 0,
+          ),
+        ]);
+        expect(summary.missedFrameBuildBudget, 0);
+      });
+
+      test('when single element exactly within budget', () {
+        final FrameTimingSummarizer summary = FrameTimingSummarizer(<FrameTiming>[
+          FrameTiming(
+            buildStart: 0,
+            buildFinish: kBuildBudget.inMicroseconds,
+            vsyncStart: 0,
+            rasterStart: 0,
+            rasterFinish: 0,
+            rasterFinishWallTime: 0,
+          ),
+        ]);
+        expect(summary.missedFrameBuildBudget, 0);
+      });
+
+      test('when many missed budget', () {
+        final FrameTimingSummarizer summary = FrameTimingSummarizer(<FrameTiming>[
+          FrameTiming(
+            buildStart: 0,
+            buildFinish: 0,
+            vsyncStart: 0,
+            rasterStart: 0,
+            rasterFinish: 0,
+            rasterFinishWallTime: 0,
+          ),
+          FrameTiming(
+            buildStart: 0,
+            buildFinish: kBuildBudget.inMicroseconds,
+            vsyncStart: 0,
+            rasterStart: 0,
+            rasterFinish: 0,
+            rasterFinishWallTime: 0,
+          ),
+          FrameTiming(
+            buildStart: 0,
+            buildFinish: (kBuildBudget + const Duration(microseconds: 1)).inMicroseconds,
+            vsyncStart: 0,
+            rasterStart: 0,
+            rasterFinish: 0,
+            rasterFinishWallTime: 0,
+          ),
+          FrameTiming(
+            buildStart: 0,
+            buildFinish: (kBuildBudget + const Duration(microseconds: 2)).inMicroseconds,
+            vsyncStart: 0,
+            rasterStart: 0,
+            rasterFinish: 0,
+            rasterFinishWallTime: 0,
+          ),
+        ]);
+        expect(summary.missedFrameBuildBudget, 2);
+      });
+    });
+  });
 }