Add a Client class to json_rpc_2.

pkg/json_rpc_2/lib/error_code.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// Error codes defined in the [JSON-RPC 2.0 specificiation][spec].
 ///
 /// These codes are generally used for protocol-level communication. Most of
 /// them shouldn't be used by the application. Those that should have
 /// convenience constructors in [RpcException].
 ///
@@ skipping 16 matching lines @@
 const INVALID_PARAMS = -32602;
 
 /// An internal JSON-RPC error.
 const INTERNAL_ERROR = -32603;
 
 /// An unexpected error occurred on the server.
 ///
 /// The spec reserves the range from -32000 to -32099 for implementation-defined
 /// server exceptions, but for now we only use one of those values.
 const SERVER_ERROR = -32000;
+
+/// Returns a human-readable name for [errorCode] if it's one specified by the
+/// JSON-RPC 2.0 spec.
+///
+/// If [errorCode] isn't defined in the JSON-RPC 2.0 spec, returns null.
+String name(int errorCode) {
+  switch (errorCode) {
+    case PARSE_ERROR: return "parse error";
+    case INVALID_REQUEST: return "invalid request";
+    case METHOD_NOT_FOUND: return "method not found";
+    case INVALID_PARAMS: return "invalid parameters";
+    case INTERNAL_ERROR: return "internal error";
+    default: return null;
+  }
+}