Remove usage of dart:json.

sdk/lib/_internal/pub/lib/src/command/cache.dart

Index: sdk/lib/_internal/pub/lib/src/command/cache.dart
diff --git a/sdk/lib/_internal/pub/lib/src/command/cache.dart b/sdk/lib/_internal/pub/lib/src/command/cache.dart
index 24c51244e62d6ece22e7d7c28189d0b586b5b550..122ee1c149d9314e349a9c3f4027e4060b316c26 100644
--- a/sdk/lib/_internal/pub/lib/src/command/cache.dart
+++ b/sdk/lib/_internal/pub/lib/src/command/cache.dart
@@ -5,8 +5,8 @@
 library pub.command.cache;
 
 import 'dart:async';
+import 'dart:convert';
 import 'dart:io';
-import 'dart:json' as json;
 
 import '../command.dart';
 import '../exit_codes.dart' as exit_codes;
@@ -43,7 +43,7 @@ class CacheCommand extends PubCommand {
 
     // TODO(keertip): Add support for non-JSON format
     // and check for --format flag
-    log.message(json.stringify({'packages': packagesObj}));
+    log.message(JSON.encode({'packages': packagesObj}));
   }
 }