Fix the slow operation notification associated with PubRoot (flutter#7885)

This resolves flutter#7792

Author: jwren

flutter-idea/src/io/flutter/pub/PubRoot.java

@@ -9,6 +9,7 @@
 import com.intellij.openapi.actionSystem.CommonDataKeys;
 import com.intellij.openapi.actionSystem.LangDataKeys;
 import com.intellij.openapi.application.ApplicationManager;
+import com.intellij.openapi.application.ReadAction;
 import com.intellij.openapi.module.Module;
 import com.intellij.openapi.module.ModuleManager;
 import com.intellij.openapi.project.Project;
@@ -18,6 +19,7 @@
 import com.intellij.openapi.util.Computable;
 import com.intellij.openapi.vfs.VirtualFile;
 import com.intellij.psi.PsiFile;
+import com.intellij.util.concurrency.AppExecutorUtil;
 import com.jetbrains.lang.dart.util.DotPackagesFileUtil;
 import io.flutter.FlutterUtils;
 import org.jetbrains.annotations.NotNull;
@@ -139,13 +141,20 @@ public static PubRoot forDirectory(@Nullable VirtualFile dir) {
     if (dir == null || !dir.isDirectory() || dir.getPath().endsWith("/")) {
       return null;
     }
+    VirtualFile pubspec = null;
+    try {
+      pubspec = ReadAction.nonBlocking(() -> {
+        return dir.findChild(PUBSPEC_YAML);
+      }).submit(AppExecutorUtil.getAppExecutorService()).get();
+    } catch (Exception e) {
+      // do nothing
+    }
 
-    final VirtualFile pubspec = dir.findChild(PUBSPEC_YAML);
     if (pubspec == null || !pubspec.exists() || pubspec.isDirectory()) {
       return null;
+    } else {
+      return new PubRoot(dir, pubspec);
     }
-
-    return new PubRoot(dir, pubspec);
   }
 
   /**