Add logging system to pub and sprinkle some logging in.

utils/pub/validator.dart

Index: utils/pub/validator.dart
diff --git a/utils/pub/validator.dart b/utils/pub/validator.dart
index d5ab53c6c1874a84d7112455b489d11ade340ef0..bada90d86274a12c2758c37795cb737b9778e395 100644
--- a/utils/pub/validator.dart
+++ b/utils/pub/validator.dart
@@ -5,6 +5,7 @@
 library validator;
 
 import 'entrypoint.dart';
+import 'log.dart' as log;
 import 'io.dart';
 import 'system_cache.dart';
 import 'utils.dart';
@@ -50,19 +51,19 @@ abstract class Validator {
       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);