Add folding support for plugins and update the main tutorial page

pkg/analyzer_plugin/lib/src/utilities/folding/folding.dart

Index: pkg/analyzer_plugin/lib/src/utilities/folding/folding.dart
diff --git a/pkg/analyzer_plugin/lib/src/utilities/folding/folding.dart b/pkg/analyzer_plugin/lib/src/utilities/folding/folding.dart
new file mode 100644
index 0000000000000000000000000000000000000000..22e8b33b8d0c04fd5d05e01dfea1d4814370cf8f
--- /dev/null
+++ b/pkg/analyzer_plugin/lib/src/utilities/folding/folding.dart
@@ -0,0 +1,54 @@
+// Copyright (c) 2017, 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.
+
+import 'package:analyzer/dart/analysis/results.dart';
+import 'package:analyzer/file_system/file_system.dart';
+import 'package:analyzer/src/generated/source.dart';
+import 'package:analyzer_plugin/protocol/protocol_common.dart'
+    hide AnalysisError;
+import 'package:analyzer_plugin/utilities/folding/folding.dart';
+
+/**
+ * A concrete implementation of [DartFoldingRequest].
+ */
+class DartFoldingRequestImpl implements DartFoldingRequest {
+  @override
+  final ResourceProvider resourceProvider;
+
+  @override
+  final ResolveResult result;
+
+  /**
+   * Initialize a newly create request with the given data.
+   */
+  DartFoldingRequestImpl(this.resourceProvider, this.result);
+
+  @override
+  String get path => result.path;
+}
+
+/**
+ * A concrete implementation of [FoldingCollector].
+ */
+class FoldingCollectorImpl implements FoldingCollector {
+  /**
+   * The list of folding regions that have been collected.
+   */
+  List<FoldingRegion> regions = <FoldingRegion>[];
+
+  /**
+   * Initialize a newly created collector.
+   */
+  FoldingCollectorImpl();
+
+  @override
+  void addRange(SourceRange range, FoldingKind kind) {
+    addRegion(range.offset, range.length, kind);
+  }
+
+  @override
+  void addRegion(int offset, int length, FoldingKind kind) {
+    regions.add(new FoldingRegion(kind, offset, length));
+  }
+}