Allow linter unit tests to be run from any directory.

I frequently run unit tests from the root directory of the SDK
package. Previous to this CL, that didn't work for the `linter`
package, because the test `validate_sdk_version_map.dart` contained a
hard-coded path that only worked if the test was invoked from the
`pkg/linter` subdirectory.

With this change, the test uses the `packageRoot` getter from the
`analyzer_utilities` package, which locates the SDK's `pkg` directory
using heuristics that work from a variety of different starting
directories, including the root of the SDK repo, anywhere within
`pkg/linter`, and in a Google3 bazel test.

Change-Id: I525ad21cb5228e670dadc98c05de18bddf3912e1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/339660
Reviewed-by: Phil Quitslund <pquitslund@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

Author: stereotype441

pkg/linter/pubspec.yaml

@@ -21,6 +21,7 @@ dependencies:
 
 # Use 'any' constraints here; we get our versions from the DEPS file.
 dev_dependencies:
+  analyzer_utilities: any
   args: any
   cli_util: any
   glob: any

pkg/linter/test/validate_sdk_version_map.dart

@@ -5,14 +5,19 @@
 import 'dart:io';
 
 import 'package:analyzer/src/lint/state.dart';
+import 'package:analyzer_utilities/package_root.dart' as pkg_root;
 import 'package:linter/src/analyzer.dart';
 import 'package:linter/src/rules.dart';
+import 'package:path/path.dart' as path;
 import 'package:test/test.dart';
 import 'package:yaml/yaml.dart';
 
 void main() {
   group('check to ensure all lints have versions in `sdk.yaml`', () {
-    var sinceFile = File('tool/since/sdk.yaml').readAsStringSync();
+    var linterPkgPath =
+        path.normalize(path.join(pkg_root.packageRoot, 'linter'));
+    var sinceFile = File(path.join(linterPkgPath, 'tool', 'since', 'sdk.yaml'))
+        .readAsStringSync();
     var versionMap = loadYamlNode(sinceFile) as YamlMap;
     registerLintRules();
     var publicRules =