Add @overrides to (a lot of) analyzer.

pkg/analyzer/lib/src/string_source.dart

Index: pkg/analyzer/lib/src/string_source.dart
diff --git a/pkg/analyzer/lib/src/string_source.dart b/pkg/analyzer/lib/src/string_source.dart
index 3cfbd2e313b01bb5fc8713ad0395eb9151d6d1da..04eb0a34ce393721e8ec6bf59c60513c92fa9661 100644
--- a/pkg/analyzer/lib/src/string_source.dart
+++ b/pkg/analyzer/lib/src/string_source.dart
@@ -53,6 +53,7 @@ class StringSource extends Source {
    * Return `true` if the given [object] is a string source that is equal to
    * this source.
    */
+  @override
   bool operator ==(Object object) {
     return object is StringSource &&
         object._contents == _contents &&