add read from stdin

prettyjson.dart

@@ -2,12 +2,25 @@ 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 jsonContent = await readContent(args);
   final jsonDecoded = json.decode(jsonContent);
   JsonEncoder encoder = new JsonEncoder.withIndent('    ');
   print(encoder.convert(jsonDecoded));
 }
+
+Future<String> readContent(List<String> args) async {
+  if (args.length == 0) {
+    return readFromStdin();
+  } else {
+    return new File(args[0]).readAsString();
+  }
+}
+
+Future<String> readFromStdin() async {
+    final stdinContentLines = stdin.transform(utf8.decoder);
+    final stdinContentArray = new List<String>();
+    await for (var line in stdinContentLines) {
+      stdinContentArray.add(line);
+    }
+    return Future<String>.value(stdinContentArray.join('\n'));
+}