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

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

Index: pkg/front_end/lib/src/base/source.dart
diff --git a/pkg/front_end/lib/src/base/source.dart b/pkg/front_end/lib/src/base/source.dart
new file mode 100644
index 0000000000000000000000000000000000000000..0ed4747915f61d9d45bc041f720f635fe9cb7486
--- /dev/null
+++ b/pkg/front_end/lib/src/base/source.dart
@@ -0,0 +1,148 @@
+// 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/analysis_target.dart';
+import 'package:front_end/src/base/timestamped_data.dart';
+import 'package:front_end/src/base/uri_kind.dart';
+
+/**
+ * The interface `Source` defines the behavior of objects representing source code that can be
+ * analyzed by the analysis engine.
+ *
+ * Implementations of this interface need to be aware of some assumptions made by the analysis
+ * engine concerning sources:
+ * * Sources are not required to be unique. That is, there can be multiple instances representing
+ * the same source.
+ * * Sources are long lived. That is, the engine is allowed to hold on to a source for an extended
+ * period of time and that source must continue to report accurate and up-to-date information.
+ * Because of these assumptions, most implementations will not maintain any state but will delegate
+ * to an authoritative system of record in order to implement this API. For example, a source that
+ * represents files on disk would typically query the file system to determine the state of the
+ * file.
+ *
+ * If the instances that implement this API are the system of record, then they will typically be
+ * unique. In that case, sources that are created that represent non-existent files must also be
+ * retained so that if those files are created at a later date the long-lived sources representing
+ * those files will know that they now exist.
+ */
+abstract class Source implements AnalysisTarget {
+  /**
+   * An empty list of sources.
+   */
+  static const List<Source> EMPTY_LIST = const <Source>[];
+
+  /**
+   * Get the contents and timestamp of this source.
+   *
+   * Clients should consider using the method [AnalysisContext.getContents]
+   * because contexts can have local overrides of the content of a source that the source is not
+   * aware of.
+   *
+   * @return the contents and timestamp of the source
+   * @throws Exception if the contents of this source could not be accessed
+   */
+  TimestampedData<String> get contents;
+
+  /**
+   * Return an encoded representation of this source that can be used to create a source that is
+   * equal to this source.
+   *
+   * @return an encoded representation of this source
+   * See [SourceFactory.fromEncoding].
+   */
+  String get encoding;
+
+  /**
+   * Return the full (long) version of the name that can be displayed to the user to denote this
+   * source. For example, for a source representing a file this would typically be the absolute path
+   * of the file.
+   *
+   * @return a name that can be displayed to the user to denote this source
+   */
+  String get fullName;
+
+  /**
+   * Return a hash code for this source.
+   *
+   * @return a hash code for this source
+   * See [Object.hashCode].
+   */
+  @override
+  int get hashCode;
+
+  /**
+   * Return `true` if this source is in one of the system libraries.
+   *
+   * @return `true` if this is in a system library
+   */
+  bool get isInSystemLibrary;
+
+  @override
+  Source get librarySource => null;
+
+  /**
+   * Return the modification stamp for this source, or a negative value if the
+   * source does not exist. A modification stamp is a non-negative integer with
+   * the property that if the contents of the source have not been modified
+   * since the last time the modification stamp was accessed then the same value
+   * will be returned, but if the contents of the source have been modified one
+   * or more times (even if the net change is zero) the stamps will be different.
+   *
+   * Clients should consider using the method
+   * [AnalysisContext.getModificationStamp] because contexts can have local
+   * overrides of the content of a source that the source is not aware of.
+   */
+  int get modificationStamp;
+
+  /**
+   * Return a short version of the name that can be displayed to the user to denote this source. For
+   * example, for a source representing a file this would typically be the name of the file.
+   *
+   * @return a name that can be displayed to the user to denote this source
+   */
+  String get shortName;
+
+  @override
+  Source get source => this;
+
+  /**
+   * Return the URI from which this source was originally derived.
+   *
+   * @return the URI from which this source was originally derived
+   */
+  Uri get uri;
+
+  /**
+   * Return the kind of URI from which this source was originally derived. If this source was
+   * created from an absolute URI, then the returned kind will reflect the scheme of the absolute
+   * URI. If it was created from a relative URI, then the returned kind will be the same as the kind
+   * of the source against which the relative URI was resolved.
+   *
+   * @return the kind of URI from which this source was originally derived
+   */
+  UriKind get uriKind;
+
+  /**
+   * Return `true` if the given object is a source that represents the same source code as
+   * this source.
+   *
+   * @param object the object to be compared with this object
+   * @return `true` if the given object is a source that represents the same source code as
+   *         this source
+   * See [Object.==].
+   */
+  @override
+  bool operator ==(Object object);
+
+  /**
+   * Return `true` if this source exists.
+   *
+   * Clients should consider using the method [AnalysisContext.exists] because
+   * contexts can have local overrides of the content of a source that the source is not aware of
+   * and a source with local content is considered to exist even if there is no file on disk.
+   *
+   * @return `true` if this source exists
+   */
+  bool exists();
+}