Fix an invalid variable name in DependencyQueue.

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// A back-tracking depth-first solver.
 ///
 /// Attempts to find the best solution for a root package's transitive
 /// dependency graph, where a "solution" is a set of concrete package versions.
 /// A valid solution will select concrete versions for every package reached
 /// from the root package's dependency graph, and each of those packages will
@@ skipping 486 matching lines @@
       return true;
     });
 
     return _visited.toList();
   }
 
   Future<DependencyQueue> _dependencyQueueFor(PackageId id) async {
     var pubspec;
     try {
       pubspec = await _solver.cache.getPubspec(id);
-    } on PackageNotFoundException catch (error) {
+    } on PackageNotFoundException {
       // 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, []);
     }
 
     _validateSdkConstraint(pubspec);
 
     var deps = pubspec.dependencies.toSet();
 
     if (id.isRoot) {
@@ skipping 256 matching lines @@
 ///
 /// Throws a [SolveFailure] if not.
 void _validateSdkConstraint(Pubspec pubspec) {
   if (pubspec.environment.sdkVersion.allows(sdk.version)) return;
 
   throw new BadSdkVersionException(pubspec.name,
       'Package ${pubspec.name} requires SDK version '
       '${pubspec.environment.sdkVersion} but the current SDK is '
       '${sdk.version}.');
 }