Throw instead of return null if durations is empty (#18957)

See https://github.com/flutter/flutter/pull/18881#discussion_r199285256

packages/flutter_driver/lib/src/driver/timeline_summary.dart

@@ -162,7 +162,7 @@ class TimelineSummary {
 
   double _averageInMillis(Iterable<Duration> durations) {
     if (durations.isEmpty)
-      return null;
+      throw new ArgumentError('durations is empty!');
 
     final int total = durations.fold<int>(0, (int t, Duration duration) => t + duration.inMilliseconds);
     return total / durations.length;
@@ -170,7 +170,7 @@ class TimelineSummary {
 
   double _percentileInMillis(Iterable<Duration> durations, double percentile) {
     if (durations.isEmpty)
-      return null;
+      throw new ArgumentError('durations is empty!');
 
     assert(percentile >= 0.0 && percentile <= 100.0);
     final List<double> doubles = durations.map<double>((Duration duration) => duration.inMilliseconds.toDouble()).toList();
@@ -181,7 +181,7 @@ class TimelineSummary {
 
   double _maxInMillis(Iterable<Duration> durations) {
     if (durations.isEmpty)
-      return null;
+      throw new ArgumentError('durations is empty!');
 
     return durations
         .map<double>((Duration duration) => duration.inMilliseconds.toDouble())

packages/flutter_driver/test/src/timeline_summary_test.dart

@@ -55,8 +55,11 @@ void main() {
     });
 
     group('average_frame_build_time_millis', () {
-      test('returns null when there is no data', () {
-        expect(summarize(<Map<String, dynamic>>[]).computeAverageFrameBuildTimeMillis(), isNull);
+      test('throws when there is no data', () {
+        expect(
+          () => summarize(<Map<String, dynamic>>[]).computeAverageFrameBuildTimeMillis(),
+          throwsA(predicate<ArgumentError>((ArgumentError e) => e.message == 'durations is empty!'))
+        );
       });
 
       test('computes average frame build time in milliseconds', () {
@@ -71,8 +74,11 @@ void main() {
     });
 
     group('worst_frame_build_time_millis', () {
-      test('returns null when there is no data', () {
-        expect(summarize(<Map<String, dynamic>>[]).computeWorstFrameBuildTimeMillis(), isNull);
+      test('throws when there is no data', () {
+        expect(
+          () => summarize(<Map<String, dynamic>>[]).computeWorstFrameBuildTimeMillis(),
+          throwsA(predicate<ArgumentError>((ArgumentError e) => e.message == 'durations is empty!'))
+        );
       });
 
       test('computes worst frame build time in milliseconds', () {
@@ -107,8 +113,11 @@ void main() {
     });
 
     group('average_frame_rasterizer_time_millis', () {
-      test('returns null when there is no data', () {
-        expect(summarize(<Map<String, dynamic>>[]).computeAverageFrameRasterizerTimeMillis(), isNull);
+      test('throws when there is no data', () {
+        expect(
+          () => summarize(<Map<String, dynamic>>[]).computeAverageFrameRasterizerTimeMillis(),
+          throwsA(predicate<ArgumentError>((ArgumentError e) => e.message == 'durations is empty!'))
+        );
       });
 
       test('computes average frame rasterizer time in milliseconds', () {
@@ -143,10 +152,14 @@ void main() {
     });
 
     group('worst_frame_rasterizer_time_millis', () {
-      test('returns null when there is no data', () {
-        expect(summarize(<Map<String, dynamic>>[]).computeWorstFrameRasterizerTimeMillis(), isNull);
+      test('throws when there is no data', () {
+        expect(
+          () => summarize(<Map<String, dynamic>>[]).computeWorstFrameRasterizerTimeMillis(),
+          throwsA(predicate<ArgumentError>((ArgumentError e) => e.message == 'durations is empty!'))
+        );
       });
 
+
       test('computes worst frame rasterizer time in milliseconds', () {
         expect(
             summarize(<Map<String, dynamic>>[
@@ -186,10 +199,14 @@ void main() {
     });
 
     group('percentile_frame_rasterizer_time_millis', () {
-      test('returns null when there is no data', () {
-        expect(summarize(<Map<String, dynamic>>[]).computeWorstFrameRasterizerTimeMillis(), isNull);
+      test('throws when there is no data', () {
+        expect(
+          () => summarize(<Map<String, dynamic>>[]).computePercentileFrameRasterizerTimeMillis(90.0),
+          throwsA(predicate<ArgumentError>((ArgumentError e) => e.message == 'durations is empty!'))
+        );
       });
 
+
       const List<List<int>> sequences = <List<int>>[
         <int>[1, 2, 3, 4, 5, 6, 7, 8, 9, 10],
         <int>[1, 2, 3, 4, 5],