Clean up pub's exceptions.

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

Index: sdk/lib/_internal/pub/lib/src/solver/version_solver.dart
diff --git a/sdk/lib/_internal/pub/lib/src/solver/version_solver.dart b/sdk/lib/_internal/pub/lib/src/solver/version_solver.dart
index 4c18768713a77ee8a4f5ac2682cec16dced04d95..9e0939c956e49dbd376f86ee391557c257370c5a 100644
--- a/sdk/lib/_internal/pub/lib/src/solver/version_solver.dart
+++ b/sdk/lib/_internal/pub/lib/src/solver/version_solver.dart
@@ -9,6 +9,7 @@ import "dart:convert";
 
 import 'package:stack_trace/stack_trace.dart';
 
+import '../exceptions.dart';
 import '../lock_file.dart';
 import '../log.dart' as log;
 import '../package.dart';
@@ -297,9 +298,6 @@ abstract class SolveFailure implements ApplicationException {
   /// Will be an empty collection if the failure is not specific to one package.
   final Iterable<Dependency> dependencies;
 
-  final innerError = null;
-  final innerTrace = null;
-
   String get message => toString();
 
   /// A message describing the specific kind of solve failure.