Fix time span in profiler

runtime/observatory/lib/src/elements/cpu_profile.dart

Index: runtime/observatory/lib/src/elements/cpu_profile.dart
diff --git a/runtime/observatory/lib/src/elements/cpu_profile.dart b/runtime/observatory/lib/src/elements/cpu_profile.dart
index e1d6d47ee632f8119b18e6313c5013ff309db7f2..93ece6626f651f68496b6fc9ed7393ec294b5c2c 100644
--- a/runtime/observatory/lib/src/elements/cpu_profile.dart
+++ b/runtime/observatory/lib/src/elements/cpu_profile.dart
@@ -500,7 +500,11 @@ class SampleBufferControlElement extends ObservatoryElement {
     refreshTime = new DateTime.now().toString();
     stackDepth = profile.stackDepth.toString();
     sampleRate = profile.sampleRate.toStringAsFixed(0);
-    timeSpan = formatTime(profile.timeSpan);
+    if (profile.sampleCount == 0) {
+      timeSpan = '0s';
+    } else {
+      timeSpan = formatTime(profile.timeSpan);
+    }
   }
 
   void tagSelectorChanged(oldValue) {