Move validation logic for .ci.yaml to flutter/flutter. (#161249)

Towards https://github.com/flutter/flutter/issues/160915 (future
cleanup: remove the validation from `flutter/cocoon`).

This PR effectively duplicates the logic in
https://github.com/flutter/cocoon/pull/4137, in `flutter/flutter`.

/cc @jtmcdole

dev/bots/test/ci_yaml_validation_test.dart

@@ -0,0 +1,161 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+@TestOn('vm')
+library;
+
+import 'dart:io' as io;
+
+import 'package:path/path.dart' as p;
+import 'package:source_span/source_span.dart';
+import 'package:yaml/yaml.dart';
+
+import './common.dart';
+
+void main() {
+  final String flutterRoot = () {
+    io.Directory current = io.Directory.current;
+    while (!io.File(p.join(current.path, 'DEPS')).existsSync()) {
+      if (current.path == current.parent.path) {
+        fail(
+          'Could not find flutter repository root (${io.Directory.current.path} -> ${current.path})',
+        );
+      }
+      current = current.parent;
+    }
+    return current.path;
+  }();
+
+  group('framework', () {
+    final List<_CiYamlTarget> targets = _CiYamlTarget.parseAll(p.join(flutterRoot, '.ci.yaml'));
+
+    for (final _CiYamlTarget target in targets) {
+      if (target.runIf == null || target.runIf!.isEmpty) {
+        continue;
+      }
+
+      setUp(() {
+        printOnFailure(target.span.message('One or more errors occurred validating'));
+      });
+
+      group(target.name, () {
+        test('must include .ci.yaml', () {
+          expect(
+            target.runIf,
+            contains('.ci.yaml'),
+            reason: '.ci.yaml inclusion means changes to the runIfs will trigger presubmit tests.',
+          );
+        });
+
+        test('must include DEPS', () {
+          expect(
+            target.runIf,
+            contains('DEPS'),
+            reason: 'DEPS updates (including the Dart SDK) mean presubmit tests must be run.',
+          );
+        });
+
+        test('must include the engine sources', () {
+          expect(
+            target.runIf,
+            contains('engine/**'),
+            reason: 'Engine updates means framework presubmit tests must be run.',
+          );
+        });
+      });
+    }
+  });
+
+  group('engine', () {
+    final List<_CiYamlTarget> targets = _CiYamlTarget.parseAll(
+      p.join(flutterRoot, 'engine', 'src', 'flutter', '.ci.yaml'),
+    );
+
+    for (final _CiYamlTarget target in targets) {
+      if (target.runIf == null || target.runIf!.isEmpty) {
+        continue;
+      }
+
+      setUp(() {
+        printOnFailure(target.span.message('One or more errors occurred validating'));
+      });
+
+      group(target.name, () {
+        test('must include .ci.yaml', () {
+          expect(
+            target.runIf,
+            contains('engine/src/flutter/.ci.yaml'),
+            reason: '.ci.yaml inclusion means changes to the runIfs will trigger presubmit tests.',
+          );
+        });
+
+        test('must include DEPS', () {
+          expect(
+            target.runIf,
+            contains('DEPS'),
+            reason: 'DEPS updates (including the Dart SDK) mean presubmit tests must be run.',
+          );
+        });
+      });
+    }
+  });
+}
+
+/// A minimal representation of an ostensibly well-formatted `.ci.yaml` file.
+///
+/// Due to the repository setup, it's not possible to reuse the existing
+/// specifications of this file, and since the test case is only testing a
+/// subset of the encoding, this class exposes only that subset.
+///
+/// For a discussion leading to this design decision, see
+/// <https://github.com/flutter/flutter/issues/160915>.
+///
+/// See also:
+/// - [`scheduler.proto`][1], the schema definition of the file format.
+/// - [`CI_YAML.md`][2], a human-authored description of the file format.
+/// - [`ci_yaml.dart`][3], where validation is performed (in `flutter/cocoon`).
+///
+/// [1]: https://github.com/flutter/cocoon/blob/main/app_dart/lib/src/model/proto/internal/scheduler.proto
+/// [2]: https://github.com/flutter/cocoon/blob/main/CI_YAML.md
+/// [3]: https://github.com/flutter/cocoon/blob/main/app_dart/lib/src/model/ci_yaml/ci_yaml.dart
+final class _CiYamlTarget {
+  _CiYamlTarget({required this.name, required this.span, required this.runIf});
+
+  factory _CiYamlTarget.fromYamlMap(YamlMap map) {
+    return _CiYamlTarget(
+      name: map['name'] as String,
+      span: map.span,
+      runIf: () {
+        final YamlList? runIf = map['runIf'] as YamlList?;
+        if (runIf == null) {
+          return null;
+        }
+        return runIf.cast<String>().toList();
+      }(),
+    );
+  }
+
+  /// Parses a list of targets from the provided `.ci.yaml` file [path].
+  static List<_CiYamlTarget> parseAll(String path) {
+    final YamlDocument yamlDoc = loadYamlDocument(
+      io.File(path).readAsStringSync(),
+      sourceUrl: Uri.parse(path),
+    );
+
+    final YamlMap root = yamlDoc.contents as YamlMap;
+    final YamlList targets = root['targets'] as YamlList;
+    return targets.nodes.map((YamlNode node) {
+      return _CiYamlTarget.fromYamlMap(node as YamlMap);
+    }).toList();
+  }
+
+  /// Name of the target.
+  final String name;
+
+  /// Where the target was parsed at in the `.ci.yaml` file.
+  final SourceSpan span;
+
+  /// Which lines were present in a `runIf` block, if any.
+  final List<String>? runIf;
+}