Rationalize arg handling for pub build and serve.

sdk/lib/_internal/pub/test/serve/utils.dart

Index: sdk/lib/_internal/pub/test/serve/utils.dart
diff --git a/sdk/lib/_internal/pub/test/serve/utils.dart b/sdk/lib/_internal/pub/test/serve/utils.dart
index 1f968dfe8a0877c56a7c63527fa8e0858bcf66fe..95e4da7a95c4db42a480448e65f7fd6151c3461f 100644
--- a/sdk/lib/_internal/pub/test/serve/utils.dart
+++ b/sdk/lib/_internal/pub/test/serve/utils.dart
@@ -358,6 +358,13 @@ Future expectWebSocketError(String method, Map params, errorCode,
   }, "send $method with $params to web socket and expect error $errorCode");
 }
 
+/// Validates that [root] was not bound to a port when pub serve started.
+Future expectNotServed(String root) {
+  return schedule(() {
+    expect(_ports.containsKey(root), isFalse);
+  });
+}
+
 /// The next id to use for a JSON-RPC 2.0 request.
 var _rpcId = 0;