Add @overrides to (a lot of) analyzer.

pkg/analyzer/lib/src/error.dart

Index: pkg/analyzer/lib/src/error.dart
diff --git a/pkg/analyzer/lib/src/error.dart b/pkg/analyzer/lib/src/error.dart
index 35ffb7c8dad4966c4e7a79a8964d3d9e396bf64d..166ba13aacef05bb0f2246bdc6a66048d91b74d6 100644
--- a/pkg/analyzer/lib/src/error.dart
+++ b/pkg/analyzer/lib/src/error.dart
@@ -20,6 +20,7 @@ class AnalyzerError implements Exception {
 
   String get message => toString();
 
+  @override
   String toString() {
     var builder = new StringBuffer();
 
@@ -89,5 +90,6 @@ class AnalyzerErrorGroup implements Exception {
       new UnmodifiableListView<AnalyzerError>(_errors);
 
   String get message => toString();
+  @override
   String toString() => errors.join("\n");
 }