Add logging system to pub and sprinkle some logging in.

utils/pub/version_solver.dart

Index: utils/pub/version_solver.dart
diff --git a/utils/pub/version_solver.dart b/utils/pub/version_solver.dart
index 31baa9ce9a9ee87e83d26acade99aa01cbca178e..1a84aa255bd0ba2a14adf6323e2c84823df893d5 100644
--- a/utils/pub/version_solver.dart
+++ b/utils/pub/version_solver.dart
@@ -40,6 +40,7 @@ library version_solver;
 import 'dart:json';
 import 'dart:math';
 import 'lock_file.dart';
+import 'log.dart' as log;
 import 'package.dart';
 import 'pubspec.dart';
 import 'root_source.dart';
@@ -59,7 +60,7 @@ import 'version.dart';
  */
 Future<List<PackageId>> resolveVersions(SourceRegistry sources, Package root,
     LockFile lockFile) {
-  print('Resolving dependencies...');
+  log.message('Resolving dependencies...');
   return new VersionSolver(sources, root, lockFile).solve();
 }
 
@@ -267,6 +268,8 @@ class ChangeVersion implements WorkItem {
   }
 
   Future process(VersionSolver solver) {
+    log.fine("Changing $package to version $version.");
+
     var dependency = solver.getDependency(package);
     var oldVersion = dependency.version;
     solver.setVersion(package, version);
@@ -418,6 +421,8 @@ class AddConstraint extends ChangeConstraint {
   AddConstraint(this.depender, this.ref);
 
   Future process(VersionSolver solver) {
+    log.fine("Adding $depender's constraint $ref.");
+
     var dependency = solver.getDependency(ref.name);
     var oldDependency = dependency.clone();
     dependency.placeConstraint(depender, ref);
@@ -449,6 +454,8 @@ class RemoveConstraint extends ChangeConstraint {
   RemoveConstraint(this.depender, this.dependent);
 
   Future process(VersionSolver solver) {
+    log.fine("Removing $depender's constraint ($_removed) on $dependent.");
+
     var dependency = solver.getDependency(dependent);
     var oldDependency = dependency.clone();
     _removed = dependency.removeConstraint(depender);
@@ -468,6 +475,8 @@ class UnlockPackage implements WorkItem {
   UnlockPackage(this.package);
 
   Future process(VersionSolver solver) {
+    log.fine("Unlocking ${package.name}.");
+
     solver.lockFile.packages.remove(package.name);
     return solver.getBestVersion(package).transform((best) {
       if (best == null) return null;