Add verbosity levels for warnings and errors.

lib/src/git.dart

 // Copyright (c) 2012, 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.
 
 /// Helper functionality for invoking Git.
 library pub.git;
 
 import 'dart:async';
 import 'dart:io';
 
@@ skipping 99 matching lines @@
 /// Checks whether [command] is the Git command for this computer.
 bool _tryGitCommand(String command) {
   // If "git --version" prints something familiar, git is working.
   try {
     var result = runProcessSync(command, ["--version"]);
     var regexp = new RegExp("^git version");
     return result.stdout.length == 1 && regexp.hasMatch(result.stdout.single);
   } on ProcessException catch (error, stackTrace) {
     var chain = new Chain.forTrace(stackTrace);
     // If the process failed, they probably don't have it.
-    log.message('Git command is not "$command": $error\n$chain');
+    log.error('Git command is not "$command": $error\n$chain');
     return false;
   }
 }