[analyzer] Fix _OverlayFile.writeAsBytesSync

Change-Id: I3b4d7ad2b7a781c49abebef307a44dd9b3381b37
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/510361
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>

pkg/analyzer/lib/file_system/overlay_file_system.dart

@@ -205,7 +205,10 @@ class _OverlayFile extends _OverlayResource implements File {
 
   @override
   void writeAsBytesSync(List<int> bytes) {
-    writeAsStringSync(String.fromCharCodes(bytes));
+    if (provider.hasOverlay(path)) {
+      throw FileSystemException(path, 'Cannot write a file with an overlay');
+    }
+    _file.writeAsBytesSync(bytes);
   }
 
   @override

pkg/analyzer/test/file_system/overlay_file_system_test.dart

@@ -354,6 +354,15 @@ class FileTest extends OverlayTestSupport {
     expect(file.toUri(), Uri.file(file.path));
   }
 
+  test_write_read_bytes_can_be_read_as_string() {
+    var expectedContent = "æble";
+    File file = _file(exists: true, content: expectedContent);
+    expect(file.readAsStringSync(), expectedContent);
+    var byteContent = file.readAsBytesSync();
+    file.writeAsBytesSync(byteContent);
+    expect(file.readAsStringSync(), expectedContent);
+  }
+
   test_writeAsBytesSync_withoutOverlay() {
     File file = _file(exists: true);
     var bytes = Uint8List.fromList([99, 99]);