Add support for "??" to summaries.

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 2315 matching lines @@
    */
   throwException,
 
   /**
    * Obtain two values `n` and `m` from [UnlinkedExpr.ints].  Then, starting at
    * the executable element for the expression being evaluated, if n > 0, pop to
    * the nth enclosing function element.  Then, push the mth local function of
    * that element onto the stack.
    */
   pushLocalFunctionReference,
+
+  /**
+   * Pop the top two values from the stack.  If the first value is non-null,
+   * keep it and discard the second.  Otherwise, keep the second and discard the
+   * first.
+   */
+  ifNull,
 }
 
 /**
  * Unlinked summary information about an import declaration.
  */
 abstract class UnlinkedImport extends base.SummaryClass {
   /**
    * Annotations for this import declaration.
    */
   @Id(8)
@@ skipping 710 matching lines @@
   @Id(11)
   int get visibleLength;
 
   /**
    * If a local variable, the beginning of the visible range; zero otherwise.
    */
   @informative
   @Id(12)
   int get visibleOffset;
 }