[analyzer] Enable implicit constness in null-aware elements

Part of #56989

Change-Id: I7e940bd3c7f62e183b5425178613c5506fdc0fc2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/397902
Reviewed-by: Keerti Parthasarathy <[email protected]>

Author: chloestefantsova

pkg/analyzer/lib/src/dart/ast/ast.dart

@@ -6202,6 +6202,7 @@ sealed class ExpressionImpl extends AstNodeImpl
         case ForElement():
         case MapLiteralEntry():
         case SpreadElement():
+        case NullAwareElement():
         case VariableDeclaration():
           break;
         default:

pkg/analyzer/test/src/dart/resolution/constant_test.dart

@@ -67,6 +67,48 @@ main() {}
     expect(value.getField('(super)')!.getField('f')!.toIntValue(), 42);
   }
 
+  test_constList_withNullAwareElement() async {
+    await assertErrorsInCode(r'''
+class A {
+  const A();
+  foo() {
+    return const [?A()];
+  }
+}
+''', [
+      error(StaticWarningCode.INVALID_NULL_AWARE_ELEMENT, 51, 1),
+    ]);
+    assertType(findNode.listLiteral('const ['), 'List<A>');
+  }
+
+  test_constMap_withNullAwareKey() async {
+    await assertErrorsInCode(r'''
+class A {
+  const A();
+  foo() {
+    return const {?A(): 0};
+  }
+}
+''', [
+      error(StaticWarningCode.INVALID_NULL_AWARE_MAP_ENTRY_KEY, 51, 1),
+    ]);
+    assertType(findNode.setOrMapLiteral('const {'), 'Map<A, int>');
+  }
+
+  test_constMap_withNullAwareValue() async {
+    await assertErrorsInCode(r'''
+class A {
+  const A();
+  foo() {
+    return const {0: ?A()};
+  }
+}
+''', [
+      error(StaticWarningCode.INVALID_NULL_AWARE_MAP_ENTRY_VALUE, 54, 1),
+    ]);
+    assertType(findNode.setOrMapLiteral('const {'), 'Map<int, A>');
+  }
+
   test_constNotInitialized() async {
     await assertErrorsInCode(r'''
 class B {
@@ -82,6 +124,20 @@ class C extends B {
     ]);
   }
 
+  test_constSet_withNullAwareElement() async {
+    await assertErrorsInCode(r'''
+class A {
+  const A();
+  foo() {
+    return const {?A()};
+  }
+}
+''', [
+      error(StaticWarningCode.INVALID_NULL_AWARE_ELEMENT, 51, 1),
+    ]);
+    assertType(findNode.setOrMapLiteral('const {'), 'Set<A>');
+  }
+
   test_context_eliminateTypeVariables() async {
     await assertNoErrorsInCode(r'''
 class A<T> {