Add an incremental reloader example and a utility tool to trigger a reload by

pkg/front_end/lib/src/incremental/file_state.dart

 // 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 'dart:async';
 import 'dart:typed_data';
 
 import 'package:crypto/crypto.dart';
 import 'package:front_end/file_system.dart';
 import 'package:front_end/src/base/resolve_relative_uri.dart';
@@ skipping 203 matching lines @@
   }
 }
 
 /// Information about known file system state.
 class FileSystemState {
   final FileSystem fileSystem;
   final TranslateUri uriTranslator;
 
   _FileSystemView _fileSystemView;
 
-  /// Mapping from file URIs to corresponding [FileState]s.
+  /// Mapping from import URIs to corresponding [FileState]s. For example, this
+  /// may contain an entry for `dart:core`.
   final Map<Uri, FileState> _uriToFile = {};
 
-  /// Mapping from file URIs to corresponding [FileState]s.
+  /// Mapping from file URIs to corresponding [FileState]s. This map should only
+  /// contain `file:*` URIs as keys.
   final Map<Uri, FileState> _fileUriToFile = {};
 
   FileSystemState(this.fileSystem, this.uriTranslator);
 
   /// Return the [FileSystem] that is backed by this [FileSystemState].  The
   /// files in this [FileSystem] always have the same content as the
   /// corresponding [FileState]s, thus avoiding race conditions when a file
   /// is updated on the actual file system.
   FileSystem get fileSystemView {
     return _fileSystemView ??= new _FileSystemView(this);
   }
 
+  /// The `file:` URI of all files currently tracked by this instance.
+  Iterable<Uri> get fileUris => _fileUriToFile.keys;
+
   /// Return the [FileState] for the given [absoluteUri], or `null` if the
   /// [absoluteUri] cannot be resolved into a file URI.
   ///
   /// The returned file has the last known state since it was last refreshed.
   Future<FileState> getFile(Uri absoluteUri) async {
     // Resolve the absolute URI into the absolute file URI.
     Uri fileUri;
     if (absoluteUri.isScheme('file')) {
       fileUri = absoluteUri;
     } else {
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       node.isEvaluated = true;
       cycle.libraries.add(node.file);
     }
     topologicallySortedCycles.add(cycle);
   }
 
   _LibraryNode getNode(FileState file) {
     return nodesOfFiles.putIfAbsent(file, () => new _LibraryNode(this, file));
   }
 }