Refactor TestContext constructor for clarity

[elliette]

Work towards #1818:

• "Make nullsafety mode a required option in test context set up"

And work towards #1845:

• "Make it clearer what parameters should be passed to TestContext (eg, it is unclear the difference between path vs pathToServe). Also the default Dart entry file is in append_body, but the default HTML entry file is in hello_world."

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Note: Core of the change is in context.dart and utilities.dart, all other changes are to test files to use the new TestContext constructors.

Refactors the TestContext constructor so that there are 2 named constructors:

• TestContext.withSoundNullSafety()
• TestContext.withWeakNullSafety()

Also changes the parameters to the TestContext constructors from:

• String directory eg "../fixtures/_testPackageSound"
• String pathToServe eg "web"
• String entry eg "../fixtures/_testPackageSound/web"
• String path eg "index.html"

to:

• String packageName eg "_testPackageSound"
• String webAssetsPath eg "web/hello_world"
• String dartEntryFileName eg "main.dart"
• String htmlEntryFileName eg "index.html"

This way all the parameters are self-consistent (no way to pass "../fixtures/_testPackageSound" and "../fixtures/_testPackage/web" for example).

Moves the fields being initialized in the constructor into getters.

Finally, makes the absolutePath function in utilities.dart more flexible by allowing clients to specify pathFromWebdev or pathFromFixtures along with pathFromDwds.

[annagrin]

Thanks Elliott! Generally LGTM with some comments where I am not sure it will work on windows, but the tests would catch them I believe.

[annagrin]

Is this only used in a uri? there are more places like this.

[elliette]

It is only a web uri. I've added a helper webCompatiblePath for other places, though not here since this is a default param and therefore needs to be const.

[annagrin]

should we use p.join(workingDirectory, '.dart_tool', 'package_config.json') so the separators are consistent?

[elliette]

Done!