Add an option to Allocator to resolve relative imports

[matanlurey]

As of right now, invalid code is produced when referring to files outside of lib/:

test/
  foo_test.dart
  foo_test.g.dart // <-- Generated

In foo_test.g.dart, an import is added like:

import 'asset:foo/test/foo_test.dart';

It should instead be:

import 'foo_test.dart';

I've solved this elsewhere using a relativeTo parameter:

  /// Returns a [Uri] for this path that can be used in a Dart import statement.
  Uri toDartUri({Uri relativeTo}) {
    if (relativeTo != null) {
      // Attempt to construct relative import.
      Uri normalizedBase = relativeTo.normalizePath();
      List<String> baseSegments = pkg_path.split(normalizedBase.path)
        ..removeLast();
      List<String> targetSegments = pkg_path.split(toAbsoluteUri().path);
      if (baseSegments.first == targetSegments.first &&
          baseSegments[1] == targetSegments[1]) {
        // Ok, we're in the same package and in the same top-level directory.
        String relativePath = pkg_path.relative(
            targetSegments.skip(2).join('/'),
            from: baseSegments.skip(2).join('/'));
        return new Uri(path: relativePath);
      }
    }

    var pathSegments = pkg_path.split(path);

    if (pathSegments.first != 'lib') {
      throw new StateError(
          'Cannot construct absolute import URI from ${relativeTo} '
          'to a non-lib Dart file: ${toAbsoluteUri()}');
    }

    var packagePath = pkg_path.joinAll(pathSegments.sublist(1));
    return new Uri(
        scheme: isDartSdk ? _dartPackage : 'package',
        path: isDartSdk ? path : '$package/$packagePath');
  }

[matanlurey]

Not necessary to solve in code_builder for now.