Marked some standalone/vmservice/* tests as flaky

tests/standalone/standalone.status

Index: tests/standalone/standalone.status
diff --git a/tests/standalone/standalone.status b/tests/standalone/standalone.status
index c0d33828576fcdd2972625ee8b3804872b772595..178c13e10e447cf3418f823b46816cd615c31de6 100644
--- a/tests/standalone/standalone.status
+++ b/tests/standalone/standalone.status
@@ -11,6 +11,10 @@ package/invalid_uri_test: Fail, OK # Fails intentionally
 
 [ $runtime == vm ]
 package/package_isolate_test: Fail # http://dartbug.com/7520.
+vmservice/isolate_echo_test: Pass, Fail # Issue: 12294
+vmservice/isolate_list_test: Pass, Fail # Issue: 12294
+vmservice/isolate_stacktrace_command_test: Pass, Fail # Issue: 12294
+vmservice/unknown_isolate_command_test: Pass, Fail # Issue: 12294
 
 [ $runtime == vm && $checked ]
 # These tests have type errors on purpose.