Add file identifiers to summaries.

pkg/analyzer/lib/src/summary/base.dart

 // Copyright (c) 2014, 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.
 
 /**
  * Base functionality which code generated summary classes are built upon.
  */
 library analyzer.src.summary.base;
 
 /**
  * Annotation used in the summary IDL to indicate the id of a field.  The set
  * of ids used by a class must cover the contiguous range from 0 to N-1, where
  * N is the number of fields.
  *
  * In order to preserve forwards and backwards compatibility, id numbers must
  * be stable between releases.  So when new fields are added they should take
  * the next available id without renumbering other fields.
  */
 class Id {
   final int value;
 
   const Id(this.value);
 }
 
 /**
+ * Annotation used in the summary IDL to indicate that a summary class can be
+ * the top level object in an encoded summary.
+ */
+class TopLevel {
+  /**
+   * If non-null, identifier that will be stored in bytes 4-7 of the file,
+   * prior all other file data.  Must be exactly 4 Latin1 characters.
+   */
+  final String fileIdentifier;
+
+  const TopLevel([this.fileIdentifier]);
+}
+
+/**
  * Instances of this class represent data that has been read from a summary.
  */
 abstract class SummaryClass {
   /**
    * Translate the data in this class into a map whose keys are the names of
    * fields and whose values are the data stored in those fields.
    *
    * Intended for testing and debugging only.
    */
   Map<String, Object> toMap();
 
   /**
    * Translate the data in this class into a JSON map, whose keys are the names
    * of fields and whose values are the data stored in those fields,
    * recursively converted into JSON.
    *
    * Fields containing their default value are elided.
    *
    * Intended for testing and debugging only.
    */
   Map<String, Object> toJson();
 }