Reserve element 0 in the references table for `dynamic`.

pkg/analyzer/tool/summary/idl.dart

 // 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 is an "idl" style description of the summary format.  It is not
  * executed directly; instead it is parsed and transformed into code that
  * implements the summary format.
  *
  * The code generation process introduces the following non-typical semantics:
@@ skipping 623 matching lines @@
 /**
  * Unlinked summary information about a compilation unit ("part file").
  */
 class UnlinkedUnit {
   /**
    * Name of the library (from a "library" declaration, if present).
    */
   String libraryName;
 
   /**
-   * Top level and prefixed names referred to by this compilation unit.
+   * Top level and prefixed names referred to by this compilation unit.  The
+   * zeroth element of this array is always populated and always represents a
+   * reference to the pseudo-type "dynamic".
    */
   List<UnlinkedReference> references;
 
   /**
    * Classes declared in the compilation unit.
    */
   List<UnlinkedClass> classes;
 
   /**
    * Enums declared in the compilation unit.
@@ skipping 65 matching lines @@
   /**
    * Indicates whether the variable is declared using the `const` keyword.
    */
   bool isConst;
 
   /**
    * Indicates whether this variable lacks an explicit type declaration.
    */
   bool hasImplicitType;
 }