Add support for "??" to 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".
 
 
 /**
  * Enum used to indicate the kind of a name in index.
@@ skipping 740 matching lines @@
    * Pop the top value from the stack and raise an exception with this value.
    */
   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
+  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
 }
 
 /**
  * Enum used to indicate the kind of a parameter.
  */
 enum UnlinkedParamKind : byte {
   /**
    * Parameter is required.
    */
   required,
@@ skipping 1883 matching lines @@
 
   /**
    * If a local variable, the beginning of the visible range; zero otherwise.
    */
   visibleOffset:uint (id: 12);
 }
 
 root_type PackageBundle;
 
 file_identifier "PBdl";