Add support for pub install.

utils/pub/sdk_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 package source that uses libraries from the Dart SDK.
+ *
+ * This currently uses the "sdkdir" command-line argument to find the SDK.
+ */
+// TODO(nweiz): This should read the SDK directory from an environment variable
+// once we can set those for tests.
+class SdkSource extends Source {
+  final String name = "sdk";
+  final bool shouldCache = false;
+
+  /**
+   * The root directory of the Dart SDK.
+   */
+  final String rootDir;
+
+  SdkSource(this.rootDir);
+
+  /**
+   * Since all the SDK files are already available locally, installation just
+   * involves symlinking the SDK library into the packages directory.
+   */
+  Future<bool> install(PackageId id, String destPath) {
+    var sourcePath = join(rootDir, "lib", id.fullName);
+    return exists(sourcePath).chain((exists) {
+      if (!exists) return new Future<bool>.immediate(false);
+      return createSymlink(sourcePath, destPath).transform((_) => true);
+    });
+  }
+}