Request: a way to access the contents of a .packages file, if provided

[sethladd]

The Dart platform now supports a .packages file as a way to specify a mapping between a package name and a location where the package's lib contents are stored.

Dart programs would like access to this map, in order to introspect package contents at runtime.

This request is for a new API added to the Isolate class. Something like this strawman:

class Isolate {
  /// Returns a map of package name to resolved URIs,
  /// if the runtime was configured with a mapping. May
  /// return null.
  ///
  /// Generally, Dart applications are configured with
  /// either a packageRoot or a packageMap, though
  /// both may return non-null values.
  static Map get packageMap;
}

[sethladd]

cc @iposva-google

[sethladd]

Dupe of #24022