Add a Pub source for pub.dartlang.org.

utils/pub/pubspec.dart

Index: utils/pub/pubspec.dart
diff --git a/utils/pub/pubspec.dart b/utils/pub/pubspec.dart
index d038779acd5ffc17cf125ddb657aba43926b141d..6fe4ff8cba8c4930052e18dc6d486f79b2331bec 100644
--- a/utils/pub/pubspec.dart
+++ b/utils/pub/pubspec.dart
@@ -59,7 +59,11 @@ class Pubspec {
       }
 
       dependencyEntries.forEach((name, spec) {
-        var description, source, versionConstraint;
+        var description, source;
+        // TODO(nweiz): The correct default for versionConstraint is new
+        // VersionRange(), but that won't work until the version solver is
+        // hooked in.
+        var versionConstraint = Version.none;
         if (spec == null) {
           description = name;
           source = sources.defaultSource;