Print the version number.

bin/sass.dart

@@ -2,6 +2,9 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
+import 'dart:async';
+import 'dart:isolate';
+
 import 'package:args/args.dart';
 import 'package:stack_trace/stack_trace.dart';
 import 'package:path/path.dart' as p;
@@ -24,9 +27,19 @@ void main(List<String> args) {
         abbr: 'c', help: 'Whether to emit terminal colors.', defaultsTo: true)
     ..addFlag('trace', help: 'Print full Dart stack traces for exceptions.')
     ..addFlag('help',
-        abbr: 'h', help: 'Print this usage information.', negatable: false);
+        abbr: 'h', help: 'Print this usage information.', negatable: false)
+    ..addFlag('version',
+        help: 'Print the version of Dart Sass.', negatable: false);
   var options = argParser.parse(getArguments(args));
 
+  if (options['version']) {
+    _loadVersion().then((version) {
+      print(version);
+      exit(0);
+    });
+    return;
+  }
+
   if (options['help'] || options.rest.isEmpty) {
     print("Compile Sass to CSS.\n");
     print("Usage: sass <input>\n");
@@ -56,3 +69,16 @@ void main(List<String> args) {
     exit(1);
   }
 }
+
+/// Loads and returns the current version of Sass.
+Future<String> _loadVersion() async {
+  var version = const String.fromEnvironment('version');
+  if (version != null) return version;
+
+  var libDir = p.fromUri(
+      await Isolate.resolvePackageUri(Uri.parse('package:sass/')));
+  var pubspec = readFile(p.join(libDir, '..', 'pubspec.yaml'));
+  return pubspec.split("\n")
+      .firstWhere((line) => line.startsWith('version: '))
+      .split(" ").last;
+}

pubspec.yaml

@@ -22,3 +22,4 @@ dev_dependencies:
   js: "^0.6.0"
   node_preamble: "^1.0.0"
   test: "^0.12.0"
+  yaml: "^2.0.0"

tool/grind.dart

@@ -7,6 +7,7 @@ import 'dart:isolate';
 
 import 'package:grinder/grinder.dart';
 import 'package:node_preamble/preamble.dart' as preamble;
+import 'package:yaml/yaml.dart';
 
 main(args) => grind(args);
 
@@ -29,7 +30,8 @@ js() {
   _ensureBuild();
   var destination = new File('build/sass.dart.js');
   Dart2js.compile(new File('bin/sass.dart'),
-      outFile: destination, extraArgs: ['-Dnode=true']);
+      outFile: destination,
+      extraArgs: ['-Dnode=true', '-Dversion=${_loadVersion()}']);
   var text = destination.readAsStringSync();
   destination.writeAsStringSync("${preamble.getPreamble()}\n$text");
 }
@@ -38,3 +40,7 @@ js() {
 void _ensureBuild() {
   new Directory('build').createSync(recursive: true);
 }
+
+/// Loads the version number from pubspec.yaml.
+String _loadVersion() =>
+    loadYaml(new File('pubspec.yaml').readAsStringSync())['version'];