Don't try to look up the root package in the default source.

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 793b1f9fd017ac19687c1230a853e9b593063a95..4550dde8f22e73cdb76a210c47e1a9dd00e44d74 100644
--- a/utils/pub/version_solver.dart
+++ b/utils/pub/version_solver.dart
@@ -54,27 +54,29 @@
  */
 Future<Map<String, Version>> resolveVersions(
     SourceRegistry sources, Package root) {
-  return new VersionSolver(sources).solve(root);
+  return new VersionSolver(sources, root).solve();
 }
 
 class VersionSolver {
   final SourceRegistry _sources;
+  final Package _root;
   final PubspecCache _pubspecs;
   final Map<String, Dependency> _packages;
   final Queue<WorkItem> _work;
   int _numIterations = 0;
 
-  VersionSolver(SourceRegistry sources)
+  VersionSolver(SourceRegistry sources, Package root)
       : _sources = sources,
+        _root = root,
         _pubspecs = new PubspecCache(sources),
         _packages = <Dependency>{},
         _work = new Queue<WorkItem>();
 
-  Future<Map<String, Version>> solve(Package root) {
+  Future<Map<String, Version>> solve() {
     // Kick off the work by adding the root package at its concrete version to
     // the dependency graph.
-    _pubspecs.cache(root);
-    enqueue(new ChangeConstraint('(entrypoint)', root.name, root.version));
+    _pubspecs.cache(_root);
+    enqueue(new ChangeConstraint('(entrypoint)', _root.name, _root.version));
 
     Future processNextWorkItem(_) {
       while (true) {
@@ -269,6 +271,13 @@ class ChangeConstraint implements WorkItem {
       return null;
     }
 
+    // If the dependency is on the root package, then we don't need to do
+    // anything since it's already at the best version.
+    if (dependent == solver._root.name) {
+      solver.enqueue(new ChangeVersion(dependent, solver._root.version));
+      return null;
+    }
+
     // The constraint has changed, so see what the best version of the package
     // that meets the new constraint is.
     // TODO(rnystrom): Should this always be the default source?