Take Sam Elkhateeb's path for "path" dependencies and clean it up some.

utils/pub/path_source.dart

+// 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.
+
+library path_source;
+
+import 'dart:async';
+import 'dart:io';
+
+import '../../pkg/path/lib/path.dart' as path;
+
+import 'io.dart';
+import 'package.dart';
+import 'pubspec.dart';
+import 'version.dart';
+import 'source.dart';
+import 'utils.dart';
+
+// TODO(rnystrom): Support relative paths. (See comment in _validatePath().)
+/// A package [Source] that installs packages from a given local file path.
+class PathSource extends Source {
+  final name = 'path';
+  final shouldCache = false;
+
+  Future<Pubspec> describe(PackageId id) {
+    return defer(() {
+      _validatePath(id.name, id.description);
+      return new Pubspec.load(id.name, id.description, systemCache.sources);
+    });
+  }
+
+  Future<bool> install(PackageId id, String path) {
+    return defer(() {
+      try {
+        _validatePath(id.name, id.description);
+      } on FormatException catch(err) {
+        return false;
+      }
+      return createPackageSymlink(id.name, id.description, path);
+    }).then((_) => true);
+  }
+
+  void validateDescription(description, {bool fromLockFile: false}) {
+    if (description is! String) {
+      throw new FormatException("The description must be a path string.");
+    }
+  }
+
+  /// Ensures that [dir] is a valid path. It must be an absolute path that
+  /// points to an existing directory. Throws a [FormatException] if the path
+  /// is invalid.
+  void _validatePath(String name, String dir) {
+    // Relative paths are not (currently) allowed because the user would expect
+    // them to be relative to the pubspec where the dependency appears. That in
+    // turn means that two pubspecs in different locations with the same
+    // relative path dependency could refer to two different packages. That
+    // violates pub's rule that a description should uniquely identify a
+    // package.
+    //
+    // At some point, we may want to loosen this, but it will mean tracking
+    // where a given PackageId appeared.
+    if (!path.isAbsolute(dir)) {
+      throw new FormatException(
+          "Path dependency for package '$name' must be an absolute path. "
+          "Was '$dir'.");
+    }
+
+    if (fileExists(dir)) {
+      throw new FormatException(
+          "Path dependency for package '$name' must refer to a "
+          "directory, not a file. Was '$dir'.");
+    }
+
+    if (!dirExists(dir)) {
+      throw new FormatException("Could not find package '$name' at '$dir'.");
+    }
+  }
+}