Generate the package spec from the lockfile.

lib/src/lock_file.dart

Index: lib/src/lock_file.dart
diff --git a/lib/src/lock_file.dart b/lib/src/lock_file.dart
index d79a42e2309c39e2b8a2473a3ccc7f0ce9c4c1fc..7c6d2221dd10f3df3a8a7b99d35bb7a101532dd3 100644
--- a/lib/src/lock_file.dart
+++ b/lib/src/lock_file.dart
@@ -7,6 +7,7 @@ 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';
@@ -152,6 +153,31 @@ class LockFile {
     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