Compile with dart2js and dart2wasm

[simolus3]

This adds support for compiling with multiple compilers, typically with dart2js and dart2wasm. The updated readme explains how this works, but this essentially boils down to:

targets:
  $default:
    builders:
      build_web_compilers:entrypoint:
        options:
          compilers:
            dart2js:
              args:
                - O2
            dart2wasm:
              args:
                - O3

This doesn't change the defaults yet, we continue to use ddc for dev builds and dart2js for release builds.

Closes #3730.

[pattobrien]

Any chance that this work would/could allow for custom entrypoints / preambles, e.g. for node js runtimes?

See: #3652

[simolus3]

I agree that we should support custom entrypoints, I'm not sure about the best way to allow that though. I've added a simple template as a builder option in aac8b6a, not sure if that's the best approach though.

[github-actions]

PR Health

Changelog Entry ✔️

Package	Changed Files

Changes to files need to be accounted for in their respective changelogs.

Package publish validation ✔️

Package	Version	Status
package:build	2.4.2-wip	WIP (no publish necessary)
package:build_config	1.1.2-wip	WIP (no publish necessary)
package:build_daemon	4.0.3-wip	WIP (no publish necessary)
package:build_modules	5.0.10-wip	WIP (no publish necessary)
package:build_resolvers	2.4.3-wip	WIP (no publish necessary)
package:build_runner	2.4.13-wip	WIP (no publish necessary)
package:build_runner_core	7.3.3-wip	WIP (no publish necessary)
package:build_test	2.2.3-wip	WIP (no publish necessary)
package:build_web_compilers	4.1.0-wip	WIP (no publish necessary)
package:scratch_space	1.0.3-wip	WIP (no publish necessary)

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[jakemac53]

We should probably make this an error during option parsing

[simolus3]

The readme is outdated now, I've changed it so that compilers takes precedence.

This is a bit unfortunate, but we can't make this an error because the keys from the user's build.yaml are merged with the defaults in build_web_compilers - so we can't assume that something's off just because both are set.
The current approach is to use the existing compiler option in the defaults (so that compilers never appears in the map unless explicitly enabled) and then have compilers take precedence.

I wonder if we should unify these into a single option, where compiler is either a string (old) or a map (new)? That makes it harder to be in an ambiguous state.

[jakemac53]

I think how you have it is fine. The plural reads nicer now that multiple can be configured, and it is a cleaner break.

[jakemac53]

@simolus3 let me know when this is ready to merge from your perspective and I will push the button!

[simolus3]

This is ready from my side 👍

[kevmoo]

I couldn't wait!
Thanks SO MUCH!

[kevmoo]

Gah! I shouldn't have merged before trying it

1. There is a cast error w/ Yaml I'm hitting. I have a fix.
2. Loading JS fails in Safari with (wasm loads fine in Chrome and Safari Tech preview)

[Error] Unhandled Promise Rejection: ReferenceError: Can't find variable: joinPathSegments
	resolveUrlWithSegments (main.dart.js:3)
	(anonymous function) (main.dart.js:25)
	(anonymous function) (main.dart.js:1)
	Global Code (main.dart.js:29)

[kevmoo]

See #3746 @simolus3 !

[kevmoo]

Also #3747 @simolus3

[kevmoo]

We're close!!!