[web] Ignore changes in *.ttf files in felt build watch mode (#13634)

lib/web_ui/dev/build.dart

@@ -60,6 +60,8 @@ class BuildCommand extends Command<bool> {
       PipelineWatcher(
         dir: libPath.absolute,
         pipeline: buildPipeline,
+        // Ignore font files that are copied whenever tests run.
+        ignore: (event) => event.path.endsWith('.ttf'),
       ).start();
       // Return a never-ending future.
       return Completer<bool>().future;
@@ -143,10 +145,13 @@ class Pipeline {
   }
 }
 
+typedef WatchEventPredicate = bool Function(WatchEvent event);
+
 class PipelineWatcher {
   PipelineWatcher({
     @required this.dir,
     @required this.pipeline,
+    this.ignore,
   }) : watcher = DirectoryWatcher(dir);
 
   /// The path of the directory to watch for changes.
@@ -158,6 +163,10 @@ class PipelineWatcher {
   /// Used to watch a directory for any file system changes.
   final DirectoryWatcher watcher;
 
+  /// A callback that determines whether to rerun the pipeline or not for a
+  /// given [WatchEvent] instance.
+  final WatchEventPredicate ignore;
+
   void start() {
     watcher.events.listen(_onEvent);
   }
@@ -166,6 +175,10 @@ class PipelineWatcher {
   Timer _scheduledPipeline;
 
   void _onEvent(WatchEvent event) {
+    if (ignore != null && ignore(event)) {
+      return;
+    }
+
     final String relativePath = path.relative(event.path, from: dir);
     print('- [${event.type}] ${relativePath}');