Increase the precision of the performance benchmarks. (#20159)

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

@@ -163,17 +163,15 @@ class TimelineSummary {
   double _averageInMillis(Iterable<Duration> durations) {
     if (durations.isEmpty)
       throw new ArgumentError('durations is empty!');
-
-    final int total = durations.fold<int>(0, (int t, Duration duration) => t + duration.inMilliseconds);
+    final double total = durations.fold<double>(0.0, (double t, Duration duration) => t + duration.inMicroseconds.toDouble() / 1000.0);
     return total / durations.length;
   }
 
   double _percentileInMillis(Iterable<Duration> durations, double percentile) {
     if (durations.isEmpty)
       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();
+    final List<double> doubles = durations.map<double>((Duration duration) => duration.inMicroseconds.toDouble() / 1000.0).toList();
     doubles.sort();
     return doubles[((doubles.length - 1) * (percentile / 100)).round()];
 
@@ -182,9 +180,8 @@ class TimelineSummary {
   double _maxInMillis(Iterable<Duration> durations) {
     if (durations.isEmpty)
       throw new ArgumentError('durations is empty!');
-
     return durations
-        .map<double>((Duration duration) => duration.inMilliseconds.toDouble())
+        .map<double>((Duration duration) => duration.inMicroseconds.toDouble() / 1000.0)
         .reduce(math.max);
   }