Reduce flakiness of http_client_stays_alive_test.

tests/standalone/io/http_client_stays_alive_test.dart

Index: tests/standalone/io/http_client_stays_alive_test.dart
diff --git a/tests/standalone/io/http_client_stays_alive_test.dart b/tests/standalone/io/http_client_stays_alive_test.dart
index 171eb38d4122d9645d13b1d5210658368d8a8786..d6a15873ec6ea26a266bea584c2c3cb71addf12f 100644
--- a/tests/standalone/io/http_client_stays_alive_test.dart
+++ b/tests/standalone/io/http_client_stays_alive_test.dart
@@ -54,7 +54,7 @@ void runServerProcess() {
       // NOTE: There is a slight chance this will cause flakiness, but there is
       // no other good way of testing correctness of timing-dependent code
       // form the outside.
-      if (seconds < SECONDS || (SECONDS + 10) < seconds) {
+      if (seconds < SECONDS || (SECONDS + 30) < seconds) {
         throw "Child did exit within $seconds seconds, but expected it to take "
               "roughly $SECONDS seconds.";
       }