Add support for serializing 'target[index]' expressions.

pkg/analyzer/lib/src/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
  * contains abstract classes which declare the interface for reading data from
  * summaries.  It is parsed and transformed into code that implements the
  * summary format.
  *
@@ skipping 1342 matching lines @@
    * the stack.
    */
   assignToProperty,
 
   /**
    * Pop from the stack `value`, `index` and `target`.  Perform
    * `target[index] op= value`  where `op` is the next assignment operator from
    * [UnlinkedConst.assignmentOperators].  Push `value` back into the stack.
    */
   assignToIndex,
+
+  /**
+   * Pop from the stack `index` and `target`.  Push into the stack the result
+   * of evaluation of `target[index]`.
+   */
+  extractIndex,
 }
 
 /**
  * Unlinked summary information about a constructor initializer.
  */
 abstract class UnlinkedConstructorInitializer extends base.SummaryClass {
   /**
    * If [kind] is `thisInvocation` or `superInvocation`, the arguments of the
    * invocation.  Otherwise empty.
    */
@@ skipping 1194 matching lines @@
    */
   @Id(11)
   int get visibleLength;
 
   /**
    * If a local variable, the beginning of the visible range; zero otherwise.
    */
   @Id(12)
   int get visibleOffset;
 }