Don't schedule timer if knownFilesSetChanges stream is not requested.

pkg/analyzer/lib/src/dart/analysis/file_state.dart

Index: pkg/analyzer/lib/src/dart/analysis/file_state.dart
diff --git a/pkg/analyzer/lib/src/dart/analysis/file_state.dart b/pkg/analyzer/lib/src/dart/analysis/file_state.dart
index 0e12d6fd812f85b68e3d0c82c797f4f8726bae88..2ec9f50f48f28df3996de5ecda5728b52d7c0331 100644
--- a/pkg/analyzer/lib/src/dart/analysis/file_state.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/file_state.dart
@@ -31,10 +31,10 @@ import 'package:convert/convert.dart';
 import 'package:crypto/crypto.dart';
 import 'package:front_end/src/base/api_signature.dart';
 import 'package:front_end/src/base/performace_logger.dart';
+import 'package:front_end/src/byte_store/byte_store.dart';
 import 'package:front_end/src/fasta/builder/builder.dart' as fasta;
 import 'package:front_end/src/fasta/parser/parser.dart' as fasta;
 import 'package:front_end/src/fasta/scanner.dart' as fasta;
-import 'package:front_end/src/byte_store/byte_store.dart';
 import 'package:meta/meta.dart';
 
 /**
@@ -704,6 +704,11 @@ class FileSystemState {
    */
   Timer _knownFilesSetChangesTimer;
 
+  /**
+   * Whether the [knownFilesSetChanges] stream is requested.
+   */
+  bool _knownFilesSetChangesRequested = false;
+
   /**
    * The controller for the [knownFilesSetChanges] stream.
    */
@@ -759,8 +764,15 @@ class FileSystemState {
    * Return the [Stream] that is periodically notified about changes to the
    * known files set.
    */
-  Stream<KnownFilesSetChange> get knownFilesSetChanges =>
-      _knownFilesSetChangesController.stream;
+  Stream<KnownFilesSetChange> get knownFilesSetChanges {
+    // If this is the first (and actually the only) time when the stream is
+    // requested, schedule the timer to send updates.
+    if (!_knownFilesSetChangesRequested) {
+      _knownFilesSetChangesRequested = true;
+      _scheduleKnownFilesSetChange();
+    }
+    return _knownFilesSetChangesController.stream;
+  }
 
   @visibleForTesting
   FileSystemStateTestView get test => _testView;
@@ -906,6 +918,11 @@ class FileSystemState {
   }
 
   void _scheduleKnownFilesSetChange() {
+    // Schedule the timer only if there is a client who listens the stream.
+    if (!_knownFilesSetChangesRequested) {
+      return;
+    }
+
     Duration delay = _knownFilesSetChangesDelay ?? new Duration(seconds: 1);
     _knownFilesSetChangesTimer ??= new Timer(delay, () {
       Set<String> addedFiles = _addedKnownFiles.toSet();