Hook in the version solver to pub install.

utils/pub/pubspec.dart

Index: utils/pub/pubspec.dart
diff --git a/utils/pub/pubspec.dart b/utils/pub/pubspec.dart
index 428ebe72899eea60f028514c9856d47250c3b2ee..d3a5bb8358969f383f7c7386b68a910b90f392b4 100644
--- a/utils/pub/pubspec.dart
+++ b/utils/pub/pubspec.dart
@@ -60,10 +60,7 @@ class Pubspec {
 
       dependencyEntries.forEach((name, spec) {
         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;
+        var versionConstraint = new VersionRange();
         if (spec == null) {
           description = name;
           source = sources.defaultSource;