Add FileSystemState.knownFilesSetChanges stream.

pkg/analyzer/test/src/dart/analysis/file_state_test.dart

Index: pkg/analyzer/test/src/dart/analysis/file_state_test.dart
diff --git a/pkg/analyzer/test/src/dart/analysis/file_state_test.dart b/pkg/analyzer/test/src/dart/analysis/file_state_test.dart
index 7308a02e793dbff5f85cf80acc6c7573290edb51..789604d0153c532fce76cf8d990bcfb0f2d8fdb2 100644
--- a/pkg/analyzer/test/src/dart/analysis/file_state_test.dart
+++ b/pkg/analyzer/test/src/dart/analysis/file_state_test.dart
@@ -2,6 +2,7 @@
 // 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:convert';
 import 'dart:typed_data';
 
@@ -562,6 +563,47 @@ part 'not-a2.dart';
     expect(fileSystemState.hasUri(generatedPath), isTrue);
   }
 
+  test_knownFilesSetChanges() async {
+    fileSystemState.test.knownFilesDelay = new Duration(milliseconds: 5);
+
+    String a = _p('/test/lib/a.dart');
+    String b = _p('/test/lib/b.dart');
+    String c = _p('/test/lib/c.dart');
+    provider.newFile(
+        a,
+        r'''
+import 'b.dart';
+''');
+    provider.newFile(b, '');
+    provider.newFile(c, '');
+
+    Stream<KnownFilesSetChange> broadcastEvents =
+        fileSystemState.knownFilesSetChanges.asBroadcastStream();
+
+    FileState file = fileSystemState.getFileForPath(a);
+    {
+      KnownFilesSetChange event = await broadcastEvents.first;
+      expect(event.added, contains(a));
+      expect(event.added, contains(b));
+      expect(event.removed, isEmpty);
+    }
+
+    // Update a.dart to import c.dart and refresh.
+    // So, c.dart should be reported as added in the next change event.
+    provider.newFile(
+        a,
+        r'''
+import 'b.dart';
+import 'c.dart';
+''');
+    file.refresh();
+    {
+      KnownFilesSetChange event = await broadcastEvents.first;
+      expect(event.added, contains(c));
+      expect(event.removed, isEmpty);
+    }
+  }
+
   test_referencedNames() {
     String path = _p('/aaa/lib/a.dart');
     provider.newFile(