Avoid random stacktraces being printed from tools/test.py during test runs

tools/testing/dart/test_runner.dart

Index: tools/testing/dart/test_runner.dart
diff --git a/tools/testing/dart/test_runner.dart b/tools/testing/dart/test_runner.dart
index f9d7b998686646a48055c9a790d396d8de41f1e6..1cb1a3900edf351f6ac120afbb892f1dffb75864 100644
--- a/tools/testing/dart/test_runner.dart
+++ b/tools/testing/dart/test_runner.dart
@@ -1854,6 +1854,7 @@ class RunningProcess {
   int pid;
   OutputLog stdout = new OutputLog();
   OutputLog stderr = new OutputLog();
+  List<String> diagnostics = <String>[];
   bool compilationSkipped = false;
   Completer<CommandOutput> completer;
 
@@ -1917,46 +1918,35 @@ class RunningProcess {
 
           // Close stdin so that tests that try to block on input will fail.
           process.stdin.close();
-          void timeoutHandler() {
+          timeoutHandler() async {
             timedOut = true;
             if (process != null) {
+              var executable, arguments;
               if (io.Platform.isLinux) {
-                // Try to print stack traces of the timed out process.
-                io.Process.run('eu-stack', ['-p ${process.pid}'])
-                .then((result) {
-                  io.stdout.write(result.stdout);
-                  io.stderr.write(result.stderr);
-                })
-                .catchError(
-                    (error) => print("Error when printing stack trace: $error"))
-                .whenComplete(() {
-                  if (!process.kill()) {
-                    DebugLogger.error("Unable to kill ${process.pid}");
-                  }
-                });
+                executable = 'eu-stack';
+                arguments = ['-p ${process.pid}'];
               } else if (io.Platform.isMacOS) {
                 // Try to print stack traces of the timed out process.
                 // `sample` is a sampling profiler but we ask it sample for 1
                 // second with a 4 second delay between samples so that we only
                 // sample the threads once.
-                io.Process.run('/usr/bin/sample',
-                               ['${process.pid}', '1', '4000', '-mayDie'])
-                .then((result) {
-                  io.stdout.write(result.stdout);
-                  io.stderr.write(result.stderr);
-                })
-                .catchError(
-                    (error) => print("Error when printing stack trace: $error"))
-                .whenComplete(() {
-                  if (!process.kill()) {
-                    DebugLogger.error("Unable to kill ${process.pid}");
-                  }
-                });
-              } else {
-                if (!process.kill()) {
-                  DebugLogger.error("Unable to kill ${process.pid}");
+                executable = '/usr/bin/sample';
+                arguments = ['${process.pid}', '1', '4000', '-mayDie'];
+              }
+
+              if (executable != null) {
+                try {
+                  var result = await io.Process.run(executable, arguments);
+                  diagnostics.addAll(result.stdout.split('\n'));
+                  diagnostics.addAll(result.stderr.split('\n'));
+                } catch (error) {
+                  diagnostics.add("Unable to capture stack traces: $error");
                 }
               }
+
+              if (!process.kill()) {
+                diagnostics.add("Unable to kill ${process.pid}");
+              }
             }
           }
 
@@ -2014,6 +2004,7 @@ class RunningProcess {
         new DateTime.now().difference(startTime),
         compilationSkipped,
         pid);
+    commandOutput.diagnostics.addAll(diagnostics);
     return commandOutput;
   }