Add an enforce-lockfile command line option

Hi!

When compiling an app to production it's important to know how it will behave. Dart has a pubspec.lock file with the exact version of each dependency. pubspec.yaml may resolve different versions in local/development environment and later when the app is published. Apparently webdev doesn't have a --enforce-lockfile flag. This flag is documented in https://dart.dev/tools/pub/packages#get-dependencies-for-production.

How can we make webdev use the pubspec.lock file?

Start by tracing webdev's command-line option handling and the path it uses to resolve dependencies for production builds. Compare the requested --enforce-lockfile behavior with Dart's documented pub production behavior, then verify that the option makes webdev use pubspec.lock rather than resolving different versions.

Written by the indexing model from the issue text.