Have dartdevc support --version.

pkg/dev_compiler/lib/src/compiler/command.dart

 // Copyright (c) 2016, 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.
 
 import 'dart:io';
 import 'package:analyzer/src/command_line/arguments.dart'
     show
         defineAnalysisArguments,
         filterUnknownArguments,
         ignoreUnrecognizedFlagsFlag;
 import 'package:analyzer/src/generated/source.dart' show Source;
 import 'package:analyzer/src/summary/package_bundle_reader.dart'
     show InSummarySource;
 import 'package:args/args.dart' show ArgParser, ArgResults;
 import 'package:args/command_runner.dart' show UsageException;
 import 'package:path/path.dart' as path;
 
 import '../analyzer/context.dart' show AnalyzerOptions;
 import 'compiler.dart' show BuildUnit, CompilerOptions, ModuleCompiler;
 import 'module_builder.dart';
 
+const _binaryName = 'dartdevc';
+
 bool _verbose = false;
 
 /// Runs a single compile for dartdevc.
 ///
 /// This handles argument parsing, usage, error handling.
 /// See bin/dartdevc.dart for the actual entry point, which includes Bazel
 /// worker support.
 int compile(List<String> args, {void printFn(Object obj)}) {
   printFn ??= print;
 
   ArgResults argResults;
   AnalyzerOptions analyzerOptions;
   try {
     var parser = ddcArgParser();
     if (args.contains('--$ignoreUnrecognizedFlagsFlag')) {
       args = filterUnknownArguments(args, parser);
     }
     argResults = parser.parse(args);
     analyzerOptions = new AnalyzerOptions.fromArguments(argResults);
   } on FormatException catch (error) {
     printFn('$error\n\n$_usageMessage');
     return 64;
   }
 
   _verbose = argResults['verbose'];
-  if (argResults['help']) {
+  if (argResults['help'] || args.isEmpty) {
     printFn(_usageMessage);
     return 0;
   }
 
+  if (argResults['version']) {
+    printFn('$_binaryName version ${_getVersion()}');
+    return 0;
+  }
+
   try {
     _compile(argResults, analyzerOptions, printFn);
     return 0;
   } on UsageException catch (error) {
     // Incorrect usage, input file not found, etc.
     printFn(error);
     return 64;
   } on CompileErrorException catch (error) {
     // Code has error(s) and failed to compile.
     printFn(error);
     return 1;
   } catch (error, stackTrace) {
     // Anything else is likely a compiler bug.
     //
     // --unsafe-force-compile is a bit of a grey area, but it's nice not to
     // crash while compiling
     // (of course, output code may crash, if it had errors).
     //
     printFn('''
 We're sorry, you've found a bug in our compiler.
 You can report this bug at:
     https://github.com/dart-lang/sdk/issues/labels/area-dev-compiler
 Please include the information below in your report, along with
 any other information that may help us track it down. Thanks!
-    dartdevc arguments: ${args.join(' ')}
+    $_binaryName arguments: ${args.join(' ')}
     dart --version: ${Platform.version}
 ```
 $error
 $stackTrace
 ```''');
     return 70;
   }
 }
 
 ArgParser ddcArgParser({bool hide: true}) {
   var argParser = new ArgParser(allowTrailingOptions: true)
     ..addFlag('help',
         abbr: 'h',
-        help: 'Display this message.\n'
-            'Add --verbose to show hidden options.',
+        help: 'Display this message. Add --verbose to show hidden options.',
         negatable: false)
     ..addFlag('verbose', abbr: 'v', help: 'Verbose output.')
+    ..addFlag('version',
+        negatable: false, help: 'Print the $_binaryName version.')
     ..addFlag(ignoreUnrecognizedFlagsFlag,
         help: 'Ignore unrecognized command line flags.',
         defaultsTo: false,
         negatable: false)
     ..addOption('out',
         abbr: 'o', allowMultiple: true, help: 'Output file (required).')
     ..addOption('module-root',
-        help: 'Root module directory.\n'
+        help: 'Root module directory. '
             'Generated module paths are relative to this root.')
     ..addOption('library-root',
-        help: 'Root of source files.\n'
+        help: 'Root of source files. '
             'Generated library names are relative to this root.');
   defineAnalysisArguments(argParser, hide: hide, ddc: true);
   addModuleFormatOptions(argParser, allowMultiple: true, hide: hide);
   AnalyzerOptions.addArguments(argParser, hide: hide);
   CompilerOptions.addArguments(argParser, hide: hide);
   return argParser;
 }
 
 bool _changed(List<int> list1, List<int> list2) {
   var length = list1.length;
@@ skipping 102 matching lines @@
   }
 
   _usageException(
       'Imported file "${source.uri}" was not found as a summary or source '
       'file. Please pass in either the summary or the source file '
       'for this import.');
   return null; // unreachable
 }
 
 String get _usageMessage =>
-    'Dart Development Compiler compiles Dart into a JavaScript module.'
-    '\n\n${ddcArgParser(hide: !_verbose).usage}';
+    'The Dart Development Compiler compiles Dart sources into a JavaScript '
+    'module.\n\n'
+    'Usage: $_binaryName [options...] <sources...>\n\n'
+    '${ddcArgParser(hide: !_verbose).usage}';
+
+String _getVersion() {
+  try {
+    // This is relative to bin/snapshot, so ../..
+    String versionPath = Platform.script.resolve('../../version').toFilePath();
+    File versionFile = new File(versionPath);
+    return versionFile.readAsStringSync().trim();
+  } catch (_) {
+    // This happens when the script is not running in the context of an SDK.
+    return "<unknown>";
+  }
+}
 
 void _usageException(String message) {
   throw new UsageException(message, _usageMessage);
 }
 
 /// Thrown when the input source code has errors.
 class CompileErrorException implements Exception {
   toString() => '\nPlease fix all errors before compiling (warnings are okay).';
 }