Add support for pub install.

utils/pub/source.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.
+
+/**
+ * A source from which to install packages.
+ *
+ * Each source has many packages that it looks up using [PackageId]s. The source
+ * is responsible for installing these packages to the package cache.
+ */
+class Source {
+  /**
+   * The default [Source] from which to fetch packages if no other [Source] is
+   * specified.
+   */
+  static Source defaultSource;
+
+  /**
+   * Looks up a source based on its name.
+   */
+  static Source fromName(String name) {
+    // TODO(nweiz): add a more principled way of registering sources here once
+    // we have more than one source. Especially important for plugins.
+    if (name == 'sdk') return defaultSource;
+    throw 'Unknown source "$name"';
+  }
+
+  /**
+   * The name of the source. Should be lower-case, suitable for use in a
+   * filename, and unique accross all sources.
+   */
+  abstract String get name();
+
+  /**
+   * Whether this source's packages should be cached in Pub's global cache
+   * directory.
+   *
+   * A source should be cached if it requires network access to retrieve
+   * packages. It doesn't need to be cached if all packages are available
+   * locally.
+   */
+  abstract bool get shouldCache();
+
+  /**
+   * Installs the package identified by [id] to [path]. Returns a [Future] that
+   * completes when the installation was finished. The [Future] should resolve
+   * to true if the package was found in the source and false if it wasn't. For
+   * all other error conditions, it should complete with an exception.
+   *
+   * If [shouldCache] is true, [path] will be a path to this source's
+   * subdirectory of the [PackageCache]'s cache directory. If [shouldCache] is
+   * false, [path] will be a path to the application's "packages" directory.
+   *
+   * [path] is guaranteed not to exist, and its parent directory is guaranteed
+   * to exist.
+   */
+  abstract Future<bool> install(PackageId id, String path);
+
+  /**
+   * Returns the name of the package identified by [id]. By default, this is
+   * just `id.fullName`, but some sources (e.g. Git) may have more complicated
+   * resolution logic.
+   *
+   * This method should be light-weight. It doesn't need to validate that the
+   * given package exists.
+   *
+   * The package name should be lower-case and suitable for use in a filename.
+   * It may contain forward slashes.
+   */
+  String packageName(PackageId id) => id.fullName;
+}