add prettyjson.dart, pubspec.yaml

prettyjson.dart

@@ -0,0 +1,13 @@
+import 'dart:convert';
+import 'dart:io';
+
+main(List<String> args) async {
+  if (args.length == 0) {
+    print('File name is not assigned\nUsage: prettyjson.dart <file.json>');
+    return;
+  }
+  final jsonContent = await new File(args[0]).readAsString();
+  final jsonDecoded = json.decode(jsonContent);
+  JsonEncoder encoder = new JsonEncoder.withIndent('    ');
+  print(encoder.convert(jsonDecoded));
+}

pubspec.yaml

@@ -0,0 +1,3 @@
+name:
+  prettyjson.dart
+