Apply Yegor's idea

Author: mdebbar

lib/web_ui/dev/build.dart

@@ -27,7 +27,7 @@ class BuildCommand extends Command<bool> {
   String get name => 'build';
 
   @override
-  String get description => 'Build the web engine package.';
+  String get description => 'Build the Flutter web engine.';
 
   bool get isWatchMode => argResults['watch'];
 
@@ -141,18 +141,16 @@ class PipelineWatcher {
   }
 
   int _pipelineRunCount = 0;
-  bool _pipelineScheduled = false;
+  Timer _scheduledPipeline;
 
   void _onEvent(WatchEvent event) {
     final String relativePath = path.relative(event.path, from: dir);
     print('- [${event.type}] ${relativePath}');
-    if (_pipelineScheduled) {
-      return;
-    }
+
     _pipelineRunCount++;
-    _pipelineScheduled = true;
-    Future.delayed(const Duration(milliseconds: 100)).then((_) {
-      _pipelineScheduled = false;
+    _scheduledPipeline?.cancel();
+    _scheduledPipeline = Timer(const Duration(milliseconds: 100), () {
+      _scheduledPipeline = null;
       _runPipeline();
     });
   }
@@ -163,11 +161,7 @@ class PipelineWatcher {
       case PipelineStatus.started:
         pipeline.stop().then((_) {
           runCount = _pipelineRunCount;
-          return pipeline.start();
-        }).then((_) {
-          if (runCount == _pipelineRunCount) {
-            print('*** Done! ***');
-          }
+          pipeline.start().then((_) => _pipelineDone(runCount));
         });
         break;
 
@@ -177,12 +171,14 @@ class PipelineWatcher {
 
       default:
         runCount = _pipelineRunCount;
-        pipeline.start().then((_) {
-          if (runCount == _pipelineRunCount) {
-            print('Done!');
-          }
-        });
+        pipeline.start().then((_) => _pipelineDone(runCount));
         break;
     }
   }
+
+  void _pipelineDone(int pipelineRunCount) {
+    if (pipelineRunCount == _pipelineRunCount) {
+      print('*** Done! ***');
+    }
+  }
 }