You will have to install the webdev package using the pub dependency manager:
pub global activate webdev
This installs webdev and makes sure it is available on the command line.
Runs a development server that continuously builds a web app.
Builds a deployable version of a web app.
When you’re developing your app, webdev chooses dartdevc, which supports incremental compilation so you can quickly see the results of your edits.
When you’re building your app for deployment, webdev chooses dart2js, which uses techniques such as tree shaking to produce optimized code.
More Reading: https://webdev.dartlang.org/tools/webdev.