move duplicated core libraries setup out of Flutter embedder into Dart VM

[mraleph]

There are pieces of dart:* initialization which are repeated in both Flutter and standalone embedders (e.g. setting up various native resolvers).

This makes this code harder to update because Flutter engine needs to be updated as well and Flutter engine does not actually run full test suites against core libraries - we only execute these against standalone embedder.

This code should move to shared place inside Dart VM (or some of the helper C++ files living in the Dart SDK and used by standalone embedder).