Analysis request `getReachableSources` (#24893).

pkg/analysis_server/lib/src/services/dependencies/reachable_source_collector.dart

Index: pkg/analysis_server/lib/src/services/dependencies/reachable_source_collector.dart
diff --git a/pkg/analysis_server/lib/src/services/dependencies/reachable_source_collector.dart b/pkg/analysis_server/lib/src/services/dependencies/reachable_source_collector.dart
new file mode 100644
index 0000000000000000000000000000000000000000..a0273cea86dc04b06a0814d7a316671b480b6716
--- /dev/null
+++ b/pkg/analysis_server/lib/src/services/dependencies/reachable_source_collector.dart
@@ -0,0 +1,53 @@
+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library services.dependencies.reachable_source_collector;
+
+import 'dart:collection';
+
+import 'package:analyzer/src/generated/engine.dart';
+import 'package:analyzer/src/generated/source.dart';
+import 'package:analyzer/task/dart.dart';
+
+/// Collects reachable sources.
+class ReachableSourceCollector {
+  final Map<String, List<String>> _sourceMap =
+      new HashMap<String, List<String>>();
+
+  final Source source;
+  final AnalysisContext context;
+  ReachableSourceCollector(this.source, this.context) {
+    if (source == null) {
+      throw new ArgumentError.notNull('source');
+    }
+    if (context == null) {
+      throw new ArgumentError.notNull('context');
+    }
+  }
+
+  /// Collect reachable sources.
+  Map<String, List<String>> collectSources() {
+    _addDependencies(source);
+    return _sourceMap;
+  }
+
+  void _addDependencies(Source source) {
+
+    String sourceUri = source.uri.toString();
+
+    // Careful not to revisit.
+    if (_sourceMap[source.uri.toString()] != null) {
+      return;
+    }
+
+    List<Source> sources = <Source>[];
+    sources.addAll(context.computeResult(source, IMPORTED_LIBRARIES));
+    sources.addAll(context.computeResult(source, EXPORTED_LIBRARIES));
+
+    _sourceMap[sourceUri] =
+        sources.map((source) => source.uri.toString()).toList();
+
+    sources.forEach((s) => _addDependencies(s));
+  }
+}