Gracefully handle a missing locked version in pub get.

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 a1195a66f431e11d0c2c00aa88d142dbced86787..1944eb0135790b50a5754be5f1553031caaaeff5 100644
--- a/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
+++ b/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
@@ -487,6 +487,12 @@ class Traverser {
       }
 
       return _traverseDeps(id, new DependencyQueue(_solver, deps));
+    }).catchError((error) {
+      if (error is! PackageNotFoundException) throw error;
+
+      // We can only get here if the lockfile refers to a specific package
+      // version that doesn't exist (probably because it was yanked).
+      throw new NoVersionException(id.name, null, id.version, []);
     });
   }