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

pkg/front_end/lib/src/base/analysis_target.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.
+
+import 'package:front_end/src/base/source.dart';
+
+/**
+ * An object with which an analysis result can be associated.
+ *
+ * Clients may implement this class when creating new kinds of targets.
+ * Instances of this type are used in hashed data structures, so subtypes are
+ * required to correctly implement [==] and [hashCode].
+ */
+abstract class AnalysisTarget {
+  /**
+   * If this target is associated with a library, return the source of the
+   * library's defining compilation unit; otherwise return `null`.
+   */
+  Source get librarySource;
+
+  /**
+   * Return the source associated with this target, or `null` if this target is
+   * not associated with a source.
+   */
+  Source get source;
+}