Add support for "??" to summaries.

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

Index: pkg/analyzer/lib/src/summary/format.fbs
diff --git a/pkg/analyzer/lib/src/summary/format.fbs b/pkg/analyzer/lib/src/summary/format.fbs
index baf782af05c92e494c2877e9e9f3818633dd791c..443019ee87b406e0dfde08dc0579c1922737def2 100644
--- a/pkg/analyzer/lib/src/summary/format.fbs
+++ b/pkg/analyzer/lib/src/summary/format.fbs
@@ -758,7 +758,14 @@ enum UnlinkedExprOperation : byte {
    * 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
 }
 
 /**