Add support for "??" to summaries.

pkg/analyzer/lib/src/summary/summarize_const_expr.dart

Index: pkg/analyzer/lib/src/summary/summarize_const_expr.dart
diff --git a/pkg/analyzer/lib/src/summary/summarize_const_expr.dart b/pkg/analyzer/lib/src/summary/summarize_const_expr.dart
index 4662be8c31962fee69c0f916ebfc018185e39408..f7e6911352860daa7af3f0640de86d39da73baa5 100644
--- a/pkg/analyzer/lib/src/summary/summarize_const_expr.dart
+++ b/pkg/analyzer/lib/src/summary/summarize_const_expr.dart
@@ -486,6 +486,8 @@ abstract class AbstractConstExprSerializer {
       operations.add(UnlinkedExprOperation.lessEqual);
     } else if (operator == TokenType.PERCENT) {
       operations.add(UnlinkedExprOperation.modulo);
+    } else if (operator == TokenType.QUESTION_QUESTION) {
+      operations.add(UnlinkedExprOperation.ifNull);
     } else {
       throw new StateError('Unknown operator: $operator');
     }