Implement messages in assert() in dev_compiler.

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/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;
@@ skipping 190 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${_argParser(hide: !_verbose).usage}';
+    '\n\n${_argParser(hide: !_verbose).usage}';
 
 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).';
 }