Flow analysis: implement "why not promoted" logic for compound assignments.

Bug: #44898
Change-Id: I57a2c62b8450c313887b516d6fe2ef85fdd0d92b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/193084
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

pkg/_fe_analyzer_shared/test/flow_analysis/why_not_promoted/data/argument_type_not_assignable_nullability_error.dart

@@ -389,3 +389,15 @@ forAssignmentInitializer(C25 c, int i) {
       null: null
   });
 }
+
+class C26 {
+  int? bad;
+}
+
+compoundAssignmentRhs(C26 c) {
+  int i = 0;
+  if (c.bad == null) return;
+  i +=
+      /*analyzer.notPromoted(propertyNotPromoted(target: member:C26.bad, type: int?))*/ c
+          . /*cfe.notPromoted(propertyNotPromoted(target: member:C26.bad, type: int?))*/ bad;
+}

pkg/analyzer/lib/src/dart/resolver/assignment_expression_resolver.dart

@@ -248,6 +248,11 @@ class AssignmentExpressionResolver {
       assignedType,
       whyNotPromoted: operator == TokenType.EQ ? whyNotPromoted : null,
     );
+    if (operator != TokenType.EQ &&
+        operator != TokenType.QUESTION_QUESTION_EQ) {
+      _resolver.checkForArgumentTypeNotAssignableForArgument(node.rightHandSide,
+          whyNotPromoted: whyNotPromoted);
+    }
   }
 
   void _setRhsContext(AssignmentExpressionImpl node, DartType leftType,

pkg/analyzer/lib/src/generated/error_verifier.dart

@@ -345,12 +345,6 @@ class ErrorVerifier extends RecursiveAstVisitor<void>
   void visitAssignmentExpression(AssignmentExpression node) {
     TokenType operatorType = node.operator.type;
     Expression lhs = node.leftHandSide;
-    Expression rhs = node.rightHandSide;
-    if (operatorType == TokenType.EQ ||
-        operatorType == TokenType.QUESTION_QUESTION_EQ) {
-    } else {
-      checkForArgumentTypeNotAssignableForArgument(rhs);
-    }
     if (operatorType == TokenType.QUESTION_QUESTION_EQ) {
       _checkForDeadNullCoalesce(node.readType as TypeImpl, node.rightHandSide);
     }