Part of https://github.com/flutter/flutter/issues/127135

flutter_tool: only enable wasm compile in master channel

* Work in progress.

* Some fixes to the command line.

* Bootstrapping works.

* Change kickoff order to maximize concurrency.

* Fix analyzer errors and formatting issues.

* Fix doc comment.

* Added unit tests for some of the web targets.

* Format issue.

* Add an integration test that builds an app to wasm.

* Add a todo for depfiles.

* Formatting.

* Apparently the license header needs to say 2014.

* `file://` URIs confuse dart2wasm on Windows. Just use absolute paths.

* Update unit tests to match new path passing.

* Have a distinct build directory for wasm, and fixes for some upstream changes.