Treat the observatory service tests as opted out of NNBD.

Change-Id: I240133399323cd25f1e3ea2924c2d8415d79ba2b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/135521
Commit-Queue: Bob Nystrom <rnystrom@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Auto-Submit: Bob Nystrom <rnystrom@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

pkg/test_runner/lib/src/compiler_configuration.dart

@@ -956,6 +956,7 @@ class AnalyzerCompilerConfiguration extends CompilerConfiguration {
       "ffi_2",
       "language_2",
       "lib_2",
+      "service",
       "standalone_2"
     };