Add a Client class to json_rpc_2.

pkg/json_rpc_2/lib/error_code.dart

Index: pkg/json_rpc_2/lib/error_code.dart
diff --git a/pkg/json_rpc_2/lib/error_code.dart b/pkg/json_rpc_2/lib/error_code.dart
index 96cb909a314c1b43460526939be225fef554c672..14b77f2e09b2feb83dac3334ad69a574cd8ed034 100644
--- a/pkg/json_rpc_2/lib/error_code.dart
+++ b/pkg/json_rpc_2/lib/error_code.dart
@@ -34,3 +34,18 @@ const INTERNAL_ERROR = -32603;
 /// 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;
+  }
+}