Install before starting the server if the lockfile is out of date.

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

Index: sdk/lib/_internal/pub/test/serve/installs_first_if_no_lockfile_test.dart
diff --git a/sdk/lib/_internal/pub/test/serve/unknown_dependency_test.dart b/sdk/lib/_internal/pub/test/serve/installs_first_if_no_lockfile_test.dart
similarity index 56%
copy from sdk/lib/_internal/pub/test/serve/unknown_dependency_test.dart
copy to sdk/lib/_internal/pub/test/serve/installs_first_if_no_lockfile_test.dart
index 434a84ccfb032f54cae1b11fcfd263800789f3e4..998811354b63d47b6427ed948c44517884cc8b48 100644
--- a/sdk/lib/_internal/pub/test/serve/unknown_dependency_test.dart
+++ b/sdk/lib/_internal/pub/test/serve/installs_first_if_no_lockfile_test.dart
@@ -12,16 +12,18 @@ import 'utils.dart';
 
 main() {
   initConfig();
-  integration("responds with a 404 unknown dependencies", () {
-    d.dir(appPath, [
-      d.appPubspec()
+  integration("installs first if there is no lockfile", () {
+    d.dir("foo", [
+      d.libPubspec("foo", "0.0.1"),
+      d.libDir("foo")
     ]).create();
 
-    startPubServe();
-    requestShould404("packages/foo/nope.dart");
-    requestShould404("assets/foo/nope.png");
-    requestShould404("dir/packages/foo/nope.dart");
-    requestShould404("dir/assets/foo/nope.png");
+    d.appDir({
+      "foo": {"path": "../foo"}
+    }).create();
+
+    startPubServe(shouldInstallFirst: true);
+    requestShouldSucceed("packages/foo/foo.dart", 'main() => "foo";');
     endPubServe();
   });
 }