Don't propose fixes to update SDK sources.

pkg/analysis_server/lib/src/services/correction/fix_internal.dart

Index: pkg/analysis_server/lib/src/services/correction/fix_internal.dart
diff --git a/pkg/analysis_server/lib/src/services/correction/fix_internal.dart b/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
index cc4311647f8e294f6e5c0d6f7c63ed2400ac678b..dbe4636640de26d28f1a6287cb0888a955c8853d 100644
--- a/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
+++ b/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
@@ -1631,6 +1631,9 @@ class FixProcessor {
 
   void _addFixToElement(FixKind kind, List args, Element element) {
     Source source = element.source;
+    if (source.isInSystemLibrary) {
+      return;
+    }
     String file = source.fullName;
     int fileStamp = element.context.getModificationStamp(source);
     _addFix(kind, args, file: file, fileStamp: fileStamp);