fix(@angular-devkit/build-angular): maintain current watch files after build errors

When using the `application` and/or `browser-esbuild` builders, the current watch files
will be maintained in the event of a build error. This better ensures that files used
in the last build/rebuild that may have caused an error will continue to be watched and
refreshed when later changed. Only when a successful rebuild has occurred will any stale
watch files be removed from the watch list.

(cherry picked from commit fd2d72a)

Author: clydin

packages/angular_devkit/build_angular/src/builders/application/build-action.ts

@@ -138,7 +138,7 @@ export async function* runEsBuildBuildAction(
   }
 
   // Wait for changes and rebuild as needed
-  let previousWatchFiles = new Set(result.watchFiles);
+  const currentWatchFiles = new Set(result.watchFiles);
   try {
     for await (const changes of watcher) {
       if (options.signal?.aborted) {
@@ -154,12 +154,23 @@ export async function* runEsBuildBuildAction(
       );
 
       // Update watched locations provided by the new build result.
-      // Add any new locations
-      watcher.add(result.watchFiles.filter((watchFile) => !previousWatchFiles.has(watchFile)));
-      const newWatchFiles = new Set(result.watchFiles);
-      // Remove any old locations
-      watcher.remove([...previousWatchFiles].filter((watchFile) => !newWatchFiles.has(watchFile)));
-      previousWatchFiles = newWatchFiles;
+      // Keep watching all previous files if there are any errors; otherwise consider all
+      // files stale until confirmed present in the new result's watch files.
+      const staleWatchFiles = result.errors.length > 0 ? undefined : new Set(currentWatchFiles);
+      for (const watchFile of result.watchFiles) {
+        if (!currentWatchFiles.has(watchFile)) {
+          // Add new watch location
+          watcher.add(watchFile);
+          currentWatchFiles.add(watchFile);
+        }
+
+        // Present so remove from stale locations
+        staleWatchFiles?.delete(watchFile);
+      }
+      // Remove any stale locations if the build was successful
+      if (staleWatchFiles?.size) {
+        watcher.remove([...staleWatchFiles]);
+      }
 
       if (writeToFileSystem) {
         // Write output files