Create pkg/front_end/src/base and begin populating with general use classes.

pkg/front_end/lib/src/base/timestamped_data.dart

+// Copyright (c) 2016, 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.
+
+/**
+ * Analysis data for which we have a modification time.
+ */
+class TimestampedData<E> {
+  /**
+   * The modification time of the source from which the data was created.
+   */
+  final int modificationTime;
+
+  /**
+   * The data that was created from the source.
+   */
+  final E data;
+
+  /**
+   * Initialize a newly created holder to associate the given [data] with the
+   * given [modificationTime].
+   */
+  TimestampedData(this.modificationTime, this.data);
+}