Install before starting the server if the lockfile is out of date.

sdk/lib/_internal/pub/lib/src/command/serve.dart

Index: sdk/lib/_internal/pub/lib/src/command/serve.dart
diff --git a/sdk/lib/_internal/pub/lib/src/command/serve.dart b/sdk/lib/_internal/pub/lib/src/command/serve.dart
index fea0f1fcdf6df1bc1d6ca5696f3d4321cfb9a15f..ce8db31dd2ea7328041b23ae7a0ac9ad0e4ab491 100644
--- a/sdk/lib/_internal/pub/lib/src/command/serve.dart
+++ b/sdk/lib/_internal/pub/lib/src/command/serve.dart
@@ -37,7 +37,18 @@ class ServeCommand extends PubCommand {
     // to pub's top-level error-handling machinery.
     var completer = new Completer();
 
-    return PubPackageProvider.create(entrypoint).then((provider) {
+    return new Future.value().then((_) {
+      // The server relies on an up-to-date lockfile, so install first if
+      // needed.
+      if (!entrypoint.isLockFileUpToDate()) {
+        log.message("Dependencies have changed, installing...");
+        return entrypoint.installDependencies().then((_) {
+          log.message("Dependencies installed!");
+        });
+      }
+    }).then((_) {
+      return PubPackageProvider.create(entrypoint);
+    }).then((provider) {
       var port;
       try {
         port = int.parse(commandOptions['port']);