Add support for "??" to summaries.

pkg/analyzer/test/src/summary/resynthesize_common.dart

Index: pkg/analyzer/test/src/summary/resynthesize_common.dart
diff --git a/pkg/analyzer/test/src/summary/resynthesize_common.dart b/pkg/analyzer/test/src/summary/resynthesize_common.dart
index be1ba73935d0d1d346dc3dcf20e2d6aebb7b000d..c69ad1a46ec29eaa8997b8b231e5cf5c6e36499c 100644
--- a/pkg/analyzer/test/src/summary/resynthesize_common.dart
+++ b/pkg/analyzer/test/src/summary/resynthesize_common.dart
@@ -2303,6 +2303,12 @@ const vIdentical = (1 == 2) ? 11 : 22;
 ''');
   }
 
+  test_const_topLevel_ifNull() {
+    checkLibrary(r'''
+const vIfNull = 1 ? 2.0;

[scheglov, 2016/11/22 17:51:37]

`1 ?? 2.0`

[Paul Berry, 2016/11/22 18:02:58]

Thank you!  Fixed.

I'm surprised this didn't lead to a test failure--it looks like resynthesis
tests no longer check for errors.  I will submit a follow-up CL which fixes
that.

+''');
+  }
+
   test_const_topLevel_literal() {
     checkLibrary(r'''
 const vNull = null;