[record_use] Parse empty fields in instances

We don't store empty maps of field values in instances, which we forgot about when parsing. This fixes that and adds a ? operator at the right place.

Change-Id: I9377a4aff186d924fbdc3324a51eed79025ceafe
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/445320
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Moritz Sümmermann <mosum@google.com>

pkg/record_use/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 0.4.2
+
+- Fix empty instance parsing.
+
 ## 0.4.1
 
 - Fix bug in signature parsing.

pkg/record_use/lib/src/constant.dart

@@ -61,7 +61,7 @@ sealed class Constant {
       ),
     ),
     InstanceConstant._type => InstanceConstant(
-      fields: (value[_valueKey] as Map<String, Object?>).map(
+      fields: (value[_valueKey] as Map<String, Object?>? ?? {}).map(
         (key, value) => MapEntry(key, constants[value as int]),
       ),
     ),

pkg/record_use/pubspec.yaml

@@ -1,7 +1,7 @@
 name: record_use
 description: >
   The serialization logic and API for the usage recording SDK feature.
-version: 0.4.1
+version: 0.4.2
 repository: https://github.com/dart-lang/sdk/tree/main/pkg/record_use
 
 environment:

pkg/record_use/test/test_data.dart

@@ -73,6 +73,11 @@ final recordedUses = Recordings(
         loadingUnit: '3',
         location: Location(uri: 'lib/test3.dart'),
       ),
+      const InstanceReference(
+        instanceConstant: InstanceConstant(fields: {}),
+        loadingUnit: '3',
+        location: Location(uri: 'lib/test3.dart'),
+      ),
     ],
   },
 );
@@ -181,6 +186,9 @@ final recordedUsesJson = '''{
         "a": 13,
         "b": 14
       }
+    },
+    {
+      "type": "Instance"
     }
   ],
   "locations": [
@@ -243,6 +251,11 @@ final recordedUsesJson = '''{
           "constant_index": 15,
           "loading_unit": "3",
           "@": 2
+        },
+        {
+          "constant_index": 16,
+          "loading_unit": "3",
+          "@": 2
         }
       ]
     }