Show the dependender(s) for missing packages.

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 c04a81822cd432d501ea807d92d67cf8df114158..6229c6e47f5c5d04d4fa6aff8224a362df922b4b 100644
--- a/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
+++ b/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
@@ -593,6 +593,14 @@ class Traverser {
       }
 
       return allowed;
+    }).catchError((error, stackTrace) {
+      if (error is PackageNotFoundException) {
+        // Show the user why the package was being requested.
+        throw new DependencyNotFoundException(
+            dep.name, error, _getDependencies(dep.name));
+      }
+
+      throw error;
     });
   }