Check fs.existsSync() before running fs.statSync() (#812)

This produces non-negligible speed improvements for Node users with
lots of imports.

CHANGELOG.md

@@ -1,3 +1,7 @@
+## 1.22.12
+
+* Improve the performance of Node.js compilation involving many `@import`s.
+
 ## 1.22.11
 
 * Don't try to load unquoted plain-CSS indented-syntax imports.

lib/src/io/node.dart

@@ -156,6 +156,13 @@ String _cleanErrorMessage(_SystemError error) {
 }
 
 bool fileExists(String path) {
+  return _systemErrorToFileSystemException(() {
+    // `existsSync()` is faster than `statSync()`, but it doesn't clarify
+    // whether the entity in question is a file or a directory. Since false
+    // negatives are much more common than false positives, it works out in our
+    // favor to check this first.
+    if (!_fs.existsSync(path)) return false;
+
     try {
       return _fs.statSync(path).isFile();
     } catch (error) {
@@ -163,9 +170,17 @@ bool fileExists(String path) {
       if (systemError.code == 'ENOENT') return false;
       rethrow;
     }
+  });
 }
 
 bool dirExists(String path) {
+  return _systemErrorToFileSystemException(() {
+    // `existsSync()` is faster than `statSync()`, but it doesn't clarify
+    // whether the entity in question is a file or a directory. Since false
+    // negatives are much more common than false positives, it works out in our
+    // favor to check this first.
+    if (!_fs.existsSync(path)) return false;
+
     try {
       return _fs.statSync(path).isDirectory();
     } catch (error) {
@@ -173,6 +188,7 @@ bool dirExists(String path) {
       if (systemError.code == 'ENOENT') return false;
       rethrow;
     }
+  });
 }
 
 void ensureDir(String path) {

pubspec.yaml

@@ -1,5 +1,5 @@
 name: sass
-version: 1.22.11
+version: 1.22.12-dev
 description: A Sass implementation in Dart.
 author: Sass Team
 homepage: https://github.com/sass/dart-sass