Add a 'dart pub cache path' command

lib/src/command/cache.dart

@@ -6,6 +6,7 @@ import '../command.dart';
 import 'cache_add.dart';
 import 'cache_clean.dart';
 import 'cache_list.dart';
+import 'cache_path.dart';
 import 'cache_preload.dart';
 import 'cache_repair.dart';
 
@@ -22,6 +23,7 @@ class CacheCommand extends PubCommand {
     addSubcommand(CacheAddCommand());
     addSubcommand(CacheListCommand());
     addSubcommand(CacheCleanCommand());
+    addSubcommand(CachePathCommand());
     addSubcommand(CacheRepairCommand());
     addSubcommand(CachePreloadCommand());
   }

lib/src/command/cache_path.dart

@@ -0,0 +1,22 @@
+// Copyright (c) 2021, 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.
+
+import '../command.dart';
+import '../log.dart' as log;
+
+class CachePathCommand extends PubCommand {
+  @override
+  String get name => 'path';
+  @override
+  String get description => 'Prints the path to the global PUB_CACHE.';
+  @override
+  bool get takesArguments => false;
+
+  CachePathCommand();
+
+  @override
+  Future<void> runProtected() async {
+    log.message(cache.rootDir);
+  }
+}

test/cache/path_test.dart

@@ -0,0 +1,27 @@
+// Copyright (c) 2013, 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.
+
+import 'package:path/path.dart' as p;
+import 'package:test/test.dart';
+
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+
+void main() {
+  test('running pub cache path', () async {
+    final cache = p.join(d.sandbox, cachePath);
+    await runPub(args: ['cache', 'path'], output: cache);
+  });
+  test(
+    'running pub cache path with PUB_CACHE set prints the right path',
+    () async {
+      final envCache = p.join(d.sandbox, 'otherCache');
+      await runPub(
+        args: ['cache', 'path'],
+        environment: {'PUB_CACHE': envCache},
+        output: envCache,
+      );
+    },
+  );
+}

test/testdata/goldens/help_test/pub cache --help.txt

@@ -10,6 +10,7 @@ Usage: pub cache [arguments...]
 Available subcommands:
   add      Install a package.
   clean    Clears the global PUB_CACHE.
+  path     Prints the path to the global PUB_CACHE.
   repair   Reinstall cached packages.
 
 Run "pub help" to see global options.

test/testdata/goldens/help_test/pub cache path --help.txt

@@ -0,0 +1,11 @@
+# GENERATED BY: test/help_test.dart
+
+## Section 0
+$ pub cache path --help
+Prints the path to the global PUB_CACHE.
+
+Usage: pub cache path <subcommand> [arguments...]
+-h, --help    Print this usage information.
+
+Run "pub help" to see global options.
+