Don't load barback for "pub run" unless necessary.

lib/src/command/run.dart

 // Copyright (c) 2014, 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.
 
 library pub.command.run;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:path/path.dart' as p;
 
 import '../command.dart';
 import '../executable.dart';
 import '../io.dart';
-import '../log.dart' as log;
 import '../utils.dart';
 
 /// Handles the `run` pub command.
 class RunCommand extends PubCommand {
   String get name => "run";
-  String get description => "Run an executable from a package.\n"
-      "NOTE: We are currently optimizing this command's startup time.";
+  String get description => "Run an executable from a package.";
   String get invocation => "pub run <executable> [args...]";
   bool get allowTrailingOptions => false;
 
   RunCommand() {
     argParser.addOption("mode",
         help: 'Mode to run transformers in.\n'
               '(defaults to "release" for dependencies, "debug" for '
                 'entrypoint)');
   }
 
@@ skipping 32 matching lines @@
       mode = BarbackMode.DEBUG;
     } else {
       mode = BarbackMode.RELEASE;
     }
 
     var exitCode = await runExecutable(entrypoint, package, executable, args,
         mode: mode);
     await flushThenExit(exitCode);
   }
 }