Add int.toString() to analyzer mock SDK.

pkg/analyzer/test/src/context/mock_sdk.dart

Index: pkg/analyzer/test/src/context/mock_sdk.dart
diff --git a/pkg/analyzer/test/src/context/mock_sdk.dart b/pkg/analyzer/test/src/context/mock_sdk.dart
index 14861d8f4512d6cf90041d41aaaf4986ccd94408..985c3cd80b454c70c42afa39ece43479a7b074e6 100644
--- a/pkg/analyzer/test/src/context/mock_sdk.dart
+++ b/pkg/analyzer/test/src/context/mock_sdk.dart
@@ -209,6 +209,8 @@ abstract class int extends num {
   external static int parse(String source,
                             { int radix,
                               int onError(String source) });
+
+  String toString();
 }
 
 abstract class double extends num {