Prepare for dart 3.0 alpha breaking changes

[annagrin]

Prepare for dart 3.0 alpha breaking changes:

• Use special branch of build_web_compilers for weak null safety tests.
• Remove passing unneeded --(no)-sound-null-safety to build daemon and build_web_compilers.
  • Build daemon detects null safety mode automatically from the pubspec and dart annotations of the project.
• Add tests to verify that every test suite runs in the intended null safety mode.

Towards: #1878

[annagrin]

@jakemac53 @nshahan It looks like dart pub upgrade fails for one of the test projects that I updated to use the new weak branch of build_web_compilers - do you know by any chance what is happening there?

1 Run dart pub upgrade
2  dart pub upgrade
3  shell: /usr/bin/bash --noprofile --norc -e -o pipefail {0}
4  env:
5    PUB_ENVIRONMENT: bot.github
6    DISPLAY: :99
7    PUB_CACHE: /home/runner/.pub-cache
8    DART_HOME: /opt/hostedtoolcache/dart-sdk
9 Resolving dependencies...
10 Git error. Command: `git clone --mirror git@github.com:dart-lang/build.git /home/runner/.pub-cache/git/cache/build-21eac6ec9bab4e4e3eca5151f845d5299c0d535a`
11 stdout: 
12 stderr: Cloning into bare repository '/home/runner/.pub-cache/git/cache/build-21eac6ec9bab4e4e3eca5151f845d5299c0d535a'...
13 git@github.com: Permission denied (publickey).
14 fatal: Could not read from remote repository.
15
16 Please make sure you have the correct access rights
17 and the repository exists.
18 exit code: 128
19 Error: Process completed with exit code 69.

[jakemac53]

You need to use http style git deps afaik for it to work in a bot environment, looks like it's trying to connect via ssh