Add logging system to pub and sprinkle some logging in.

utils/pub/validator.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.
 
 library validator;
 
 import 'entrypoint.dart';
+import 'log.dart' as log;
 import 'io.dart';
 import 'system_cache.dart';
 import 'utils.dart';
 import 'validator/pubspec_field.dart';
 
 /// The base class for validators that check whether a package is fit for
 /// uploading. Each validator should override [errors], [warnings], or both to
 /// return lists of errors or warnings to display to the user. Errors will cause
 /// the package not to be uploaded; warnings will require the user to confirm
 /// the upload.
@@ skipping 25 matching lines @@
     // TODO(nweiz): The sleep 0 here forces us to go async. This works around
     // 3356, which causes a bug if all validators are (synchronously) using
     // Future.immediate and an error is thrown before a handler is set up.
     return sleep(0).chain((_) {
       return Futures.wait(validators.map((validator) => validator.validate()));
     }).transform((_) {
       var errors = flatten(validators.map((validator) => validator.errors));
       var warnings = flatten(validators.map((validator) => validator.warnings));
 
       if (!errors.isEmpty) {
-        printError("== Errors:");
+        log.error("== Errors:");
         for (var error in errors) {
-          printError("* $error");
+          log.error("* $error");
         }
-        printError("");
+        log.error("");
       }
 
       if (!warnings.isEmpty) {
-        printError("== Warnings:");
+        log.warning("== Warnings:");
         for (var warning in warnings) {
-          printError("* $warning");
+          log.warning("* $warning");
         }
-        printError("");
+        log.warning("");
       }
 
       return new Pair<List<String>, List<String>>(errors, warnings);
     });
   }
 }