Don't recompute the package graph unnecessarily in Entrypoint.

sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart

Index: sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
diff --git a/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart b/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
index d8b57614b5ca2325509445d049feb558c2e2295b..d8cc66ad53e2b255aab1592bed3f618eba89db37 100644
--- a/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
+++ b/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
@@ -143,8 +143,13 @@ class BacktrackingSolver {
       _validateSdkConstraint(root.pubspec);
       return _traverseSolution();
     }).then((packages) {
+      var pubspecs = new Map.fromIterable(packages,
+          key: (id) => id.name,
+          value: (id) => cache.getCachedPubspec(id));
+
       return new SolveResult.success(sources, root, lockFile, packages,
-          overrides, _getAvailableVersions(packages), attemptedSolutions);
+          overrides, pubspecs, _getAvailableVersions(packages),
+          attemptedSolutions);
     }).catchError((error) {
       if (error is! SolveFailure) throw error;