Add a fallback mode for building summaries.

pkg/analyzer/lib/src/summary/format.fbs

 // Copyright (c) 2015, 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.
 //
 // This file has been automatically generated.  Please do not edit it manually.
 // To regenerate the file, use the script "pkg/analyzer/tool/generate_files".
 
 
 /**
  * Kind of a source in the cache.
@@ skipping 771 matching lines @@
   /**
    * Information about entities in the export namespace of the library that are
    * not in the public namespace of the library (that is, entities that are
    * brought into the namespace via `export` directives).
    *
    * Sorted by name.
    */
   exportNames:[LinkedExportName] (id: 4);
 
   /**
+   * Indicates whether this library was summarized in "fallback mode".  If
+   * true, all other fields in the data structure have their default values.
+   */
+  fallbackMode:bool (id: 5);
+
+  /**
    * For each import in [UnlinkedUnit.imports], an index into [dependencies]
    * of the library being imported.
    */
   importDependencies:[uint] (id: 1);
 
   /**
    * The number of elements in [dependencies] which are not "linked"
    * dependencies (that is, the number of libraries in the direct imports plus
    * the transitive closure of exports, plus the library itself).
    */
@@ skipping 1132 matching lines @@
    * Offset of the type parameter name relative to the beginning of the file.
    */
   nameOffset:uint (id: 1);
 }
 
 /**
  * Unlinked summary information about a compilation unit ("part file").
  */
 table UnlinkedUnit {
   /**
+   * If this compilation unit was summarized in fallback mode, the path where
+   * the compilation unit may be found on disk.  Otherwise empty.
+   *
+   * When this field is non-empty, all other fields in the data structure have
+   * their default values.
+   */
+  fallbackModePath:string (id: 16);
+
+  /**
    * Classes declared in the compilation unit.
    */
   classes:[UnlinkedClass] (id: 2);
 
   /**
    * Code range of the unit.
    */
   codeRange:CodeRange (id: 15);
 
   /**
@@ skipping 169 matching lines @@
 
   /**
    * If a local variable, the beginning of the visible range; zero otherwise.
    */
   visibleOffset:uint (id: 12);
 }
 
 root_type PackageBundle;
 
 file_identifier "PBdl";