Generate the package spec from the lockfile.

lib/src/lock_file.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.
 
 library pub.lock_file;
 
 import 'dart:collection';
 
 import 'package:path/path.dart' as p;
+import 'package:package_config/packages_file.dart' as packages_file;
 import 'package:pub_semver/pub_semver.dart';
 import 'package:source_span/source_span.dart';
 import 'package:yaml/yaml.dart';
 
 import 'io.dart';
 import 'package.dart';
 import 'source_registry.dart';
 import 'utils.dart';
 
 /// A parsed and validated `pubspec.lock` file.
@@ skipping 125 matching lines @@
   ///
   /// Returns an identical [LockFile] if there's no package named [name].
   LockFile removePackage(String name) {
     if (!this.packages.containsKey(name)) return this;
 
     var packages = new Map.from(this.packages);
     packages.remove(name);
     return new LockFile._(packages, _sources);
   }
 
+  /// Returns the contents of the `.packages` file generated from this lockfile.
+  ///
+  /// If [entrypoint] is passed, a relative entry is added for its "lib/"
+  /// directory.
+  String packagesFile([String entrypoint]) {
+    var header = """
+Generated by pub on ${new DateTime.now()}.
+This file contains a map from Dart package names to Dart package locations.
+Dart tools, including the Dart VM and Dart analyzer, rely on the content.

[Bob Nystrom, 2015/08/07 23:51:25]

I don't think it's worth putting this in every file. How about just the first
line and drop the rest?

[nweiz, 2015/08/08 00:52:57]

Done.

+AUTO GENERATED - DO NOT EDIT
+""";
+
+    var map = new Map.fromIterable(ordered(packages.keys), value: (name) {
+      var id = packages[name];
+      var source = _sources[id.source];
+      return p.toUri(p.join(source.getDirectory(id), "lib"));

[Bob Nystrom, 2015/08/07 23:51:25]

What do you think about making this path relative to the .packages file?

[nweiz, 2015/08/08 00:52:57]

It does that for path dependencies. For cached dependencies, though, we want it
to be absolute so the package can be moved around and still have a valid
.packages file.

+    });
+
+    if (entrypoint != null) map[entrypoint] = Uri.parse("lib/");
+
+    var text = new StringBuffer();
+    packages_file.write(text, map, comment: header);
+    return text.toString();
+  }
+
   /// Returns the serialized YAML text of the lock file.
   ///
   /// [packageDir] is the containing directory of the root package, used to
   /// properly serialize package descriptions.
   String serialize(String packageDir) {
     // Convert the dependencies to a simple object.
     var data = {};
     packages.forEach((name, package) {
       var description = _sources[package.source]
           .serializeDescription(packageDir, package.description);
 
       data[name] = {
         'version': package.version.toString(),
         'source': package.source,
         'description': description
       };
     });
 
     return """
 # Generated by pub
 # See http://pub.dartlang.org/doc/glossary.html#lockfile
 ${yamlToString({'packages': data})}
 """;
   }
 }