[analyzer] Fixes dart path on Windows for DartFormat

Addresses TODO from https://dart-review.googlesource.com/c/sdk/+/448161.

Change-Id: I2debc307a176deb7670e5681af9e712e25573eb1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/448607
Auto-Submit: Felipe Morschel <[email protected]>
Reviewed-by: Paul Berry <[email protected]>
Reviewed-by: Samuel Rawlins <[email protected]>
Commit-Queue: Samuel Rawlins <[email protected]>

Author: FMorschel

pkg/analyzer_utilities/lib/tools.dart

@@ -245,9 +245,15 @@ class CodeGeneratorSettings {
 
 /// A utility class for invoking 'dart format'.
 class DartFormat {
-  static final String _dartPath =
-      // TODO(paulberry): check that this is the right path on Windows.
-      join(packageRoot, '..', 'tools', 'sdks', 'dart-sdk', 'bin', 'dart');
+  static final String _dartPath = join(
+    packageRoot,
+    '..',
+    'tools',
+    'sdks',
+    'dart-sdk',
+    'bin',
+    Platform.isWindows ? 'dart.exe' : 'dart',
+  );
 
   static void formatFile(File file) {
     var result = Process.runSync(_dartPath, ['format', file.path]);