Make service tests more resilient to isolate load timing

runtime/observatory/tests/service/test_helper.dart

Index: runtime/observatory/tests/service/test_helper.dart
diff --git a/runtime/observatory/tests/service/test_helper.dart b/runtime/observatory/tests/service/test_helper.dart
index 58162431f0d13324955ed65fed1273f799f9e5c1..e1f43af87aaf206766376a74a43ca944456440b5 100644
--- a/runtime/observatory/tests/service/test_helper.dart
+++ b/runtime/observatory/tests/service/test_helper.dart
@@ -315,7 +315,7 @@ class _ServiceTesterRunner {
 
         // Run isolate tests.
         if (isolateTests != null) {
-          var isolate = await vm.isolates.first.load();
+          var isolate = await getFirstIsolate(vm);
           var testIndex = 1;
           var totalTests = isolateTests.length;
           for (var test in isolateTests) {
@@ -341,6 +341,50 @@ class _ServiceTesterRunner {
       });
     });
   }
+
+
+
+  Future<Isolate> getFirstIsolate(WebSocketVM vm) async {
+    if (vm.isolates.isNotEmpty) {
+      var isolate = await vm.isolates.first.load();
+      if (isolate is Isolate) {
+        return isolate;
+      }
+    }
+
+    Completer completer = new Completer();
+    vm.getEventStream(VM.kIsolateStream).then((stream) {
+      var subscription;
+      subscription = stream.listen((ServiceEvent event) async {
+        if (completer == null) {
+          subscription.cancel();
+          return;
+        }
+        if (event.kind == ServiceEvent.kIsolateRunnable) {
+          if (vm.isolates.isNotEmpty) {
+            vm.isolates.first.load().then((result) {
+              if (result is Isolate) {
+                subscription.cancel();
+                completer.complete(result);
+                completer = null;
+              }
+            });
+          }
+        }
+      });
+    });
+
+    // The isolate may have started before we subscribed.
+    if (vm.isolates.isNotEmpty) {
+      vm.isolates.first.reload().then((result) async {
+        if (completer != null && result is Isolate) {
+          completer.complete(result);
+          completer = null;
+        }
+      });
+    }
+    return await completer.future;
+  }
 }
 
 /// Runs [tests] in sequence, each of which should take an [Isolate] and