[Tests] - Fix legacy tests that broke when non nullability was enabled by default

1. lib_2/isolate/issue_6610_test - the source is served from a http request, the autodetect code causes the code to be served multiple times leading to 'count' being incremented multiple times.

2. standalone_2/http_launch_test - source is served from http request and hence the CFE auto opt out does not work for the spawned isolate

3. standalone_2/io/named_pipe_script_test - serves the source using a named pipe and the autodetect code does multiple fetches of the source resulting in an error

4. standalone_2/io/test_extension_test - spawns processes from a directory that is not auto opted out by CFE

TESTS = Fixes existing legacy tests.

Change-Id: I3c78055136659850ce068d6ab254279839d2414b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/170280
Commit-Queue: Siva Annamalai <asiva@google.com>
Reviewed-by: Alexander Markov <alexmarkov@google.com>

tests/lib_2/isolate/issue_6610_test.dart

@@ -2,8 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// VMOptions=--enable-isolate-groups
-// VMOptions=--no-enable-isolate-groups
+// VMOptions=--enable-experiment=no-non-nullable --enable-isolate-groups
+// VMOptions=--enable-experiment=no-non-nullable --no-enable-isolate-groups
 
 // Testing that Isolate.spawn copies the source code of the parent isolate,
 // rather than rereading the parent's source URI.
@@ -28,6 +28,7 @@ void main() {
     server.listen((HttpRequest request) {
       ++count;
       request.response.write("""
+        // @dart = 2.9
         import 'dart:isolate';
 
         void main(_, SendPort port) {

tests/standalone_2/http_launch_data/http_launch_main.dart

@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart = 2.9
+
 library http_launch_main;
 
 import 'package:simple/simple.dart';

tests/standalone_2/io/named_pipe_script_test.dart

@@ -3,6 +3,8 @@
 // BSD-style license that can be found in the LICENSE file.
 // Testing file input stream, VM-only, standalone test.
 
+// VMOptions=--enable-experiment=no-non-nullable
+
 import "dart:convert";
 import "dart:io";
 

tests/standalone_2/io/test_extension_fail_tester.dart

@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart = 2.9
+
 library test_extension_test;
 
 import "dart:async";

tests/standalone_2/io/test_extension_tester.dart

@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart = 2.9
+
 // VMOptions=--enable-isolate-groups
 // VMOptions=--no-enable-isolate-groups