A package that defines a common class, PackageResolver, for defining how to resolve package: URIs. This class may be based on the current isolate's package resolution strategy, but it may also be explicitly defined by the user—for example, you could create a resolver that represents the strategy used to compile a .dart.js file.

The Dart VM provides two mutually exclusive means of resolving package: URIs: a package spec and a package root.

• A package spec usually comes in the form of a .packages file on the filesystem. It defines an individual root URL for each package name, so that package:$name/$path resolves to $root/$path.

• A package root is a single URL that acts as the base for all package: URIs, so that package:$name/$path resolves to $base/$name/$path.

This normalizes access to these resolution schemes, and makes it easy for code to resolve package URIs no matter where the resolution information comes from.