Make pub generate .packages file.

lib/src/package_locations.dart

+// Copyright (c) 2015, 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.
+
+/// Keeps track of locations of packages, and can create a `.packages` file.
+// TODO(lrn): Also move packages/ directory management to this library.
+library pub.package_locations;
+
+import 'dart:async';
+
+import 'package:package_config/packages_file.dart' as packages_file;
+import 'package:path/path.dart' as p;
+
+import 'package_graph.dart';
+import 'io.dart';
+import 'log.dart' as log;
+import 'utils.dart' show ordered;
+
+/// Creates a `.packages` file with the locations of the packages in [graph].
+///
+/// The file is written in the root directory of the entrypoint of [graph].
+///
+/// If the file already exists, it is deleted before the new content is written.
+void writePackagesMap(PackageGraph graph) {
+  var packagesFilePath = graph.entrypoint.root.path(".packages");
+  var content = _createPackagesMap(graph);
+  writeTextFile(packagesFilePath, content);
+}
+
+/// Template for header text put into `.packages` file.
+///
+/// Contains the literal string `$now` which should be replaced by a timestamp.
+const _headerText = r"""
+Generate by pub on $now.
+This file contains a map from Dart package names to Dart package locations.
+Dart tools, including the Dart VM and Dart analyzer, rely on the content.
+AUTO GENERATED - DO NOT EDIT
+""";
+
+/// Returns the contents of the `.packages` file created from a package graph.
+///
+/// The paths in the generated `.packages` file are always absolute URIs.
+String _createPackagesMap(PackageGraph packageGraph) {
+  var header = _headerText.replaceFirst(r"$now", new DateTime.now().toString());
+
+  var packages = packageGraph.packages;
+  var uriMap = {};
+  for (var packageName in ordered(packages.keys)) {
+    var location = packages[packageName].path("lib");
+    uriMap[packageName] = p.toUri(location);
+  }
+
+  var text = new StringBuffer();
+  packages_file.write(text, uriMap, comment: header);
+  return text.toString();
+}