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

@@ -156,23 +156,39 @@ String _cleanErrorMessage(_SystemError error) {
 }
 
 bool fileExists(String path) {
-  try {
-    return _fs.statSync(path).isFile();
-  } catch (error) {
-    var systemError = error as _SystemError;
-    if (systemError.code == 'ENOENT') return false;
-    rethrow;
-  }
+  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) {
+      var systemError = error as _SystemError;
+      if (systemError.code == 'ENOENT') return false;
+      rethrow;
+    }
+  });
 }
 
 bool dirExists(String path) {
-  try {
-    return _fs.statSync(path).isDirectory();
-  } catch (error) {
-    var systemError = error as _SystemError;
-    if (systemError.code == 'ENOENT') return false;
-    rethrow;
-  }
+  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) {
+      var systemError = error as _SystemError;
+      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