Remove dart:io's Platform.packageRoot and dart:isolate's Isolate.packageRoot APIs

[mit-mit]

The dart:io library contains a packageRoot API:

/// This returns `null`, as `packages/` directories are no longer supported.
///
@Deprecated('packages/ directory resolution is not supported in Dart 2')
static String? get packageRoot => null; // TODO(mfairhurst): remove this

Similarily in the isolate API:

/// The package root of the current isolate, if any.
///
/// This getter returns `null`, as the `packages/` directory is not supported
/// in Dart 2.
@Deprecated('packages/ directory resolution is not supported in Dart 2.')
external static Future<Uri?> get packageRoot;

As the deprecation comments state, this is not only deprecated but non-functional in Dart 2, and is hardcoded to return null. We should go ahead and remove it.

[mit-mit]

While the API doesn't currently work, removing the API is technically a breaking change, so tagging.

cc @devoncarew

[mit-mit]

CL is https://dart-review.googlesource.com/c/sdk/+/221340

[devoncarew]

@bwilkerson - would dart fix help at all here? Is there any useful guidance we can have for deleted APIs - does flutter have any similar situations?

[mit-mit]

I don't think we need to spend any more time here -- I'm fairly sure there is a very minimal risk. Even pub has supported package directories for more than two years: dart-lang/pub#1960

[mit-mit]

Fixed in 71e3dac

[zanderso]

This change is breaking the Dart roll into Flutter. See flutter/engine#30231.

[mit-mit]

Sorry about that; PR to roll Engine is flutter/engine#30236. Framework was already prepared in flutter/flutter#94603