Migration: integrate flow analysis of ?? into FixBuilder

Change-Id: I3d576c704ffd0b3b44fff0ceb442e355790ecb84
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/121342
Reviewed-by: Mike Fairhurst <mfairhurst@google.com>

Author: stereotype441

pkg/nnbd_migration/lib/src/fix_builder.dart

@@ -205,11 +205,11 @@ abstract class FixBuilder extends GeneralizingAstVisitor<DartType> {
       case TokenType.QUESTION_QUESTION:
         // If `a ?? b` is used in a non-nullable context, we don't want to
         // migrate it to `(a ?? b)!`.  We want to migrate it to `a ?? b!`.
-        // TODO(paulberry): once flow analysis supports `??`, integrate it here.
-        // See https://github.com/dart-lang/sdk/issues/38680
         var leftType = visitSubexpression(node.leftOperand,
             _typeSystem.makeNullable(_contextType as TypeImpl));
+        _flowAnalysis.ifNullExpression_rightBegin();
         var rightType = visitSubexpression(node.rightOperand, _contextType);
+        _flowAnalysis.ifNullExpression_end();
         return _typeSystem.leastUpperBound(
             _typeSystem.promoteToNonNull(leftType as TypeImpl), rightType);
       default:

pkg/nnbd_migration/test/fix_builder_test.dart

@@ -420,6 +420,17 @@ _f(int/*?*/ x, double/*?*/ y) {
     visitSubexpression(findNode.binary('??'), 'num?');
   }
 
+  test_binaryExpression_question_question_flow() async {
+    await analyze('''
+_f(int/*?*/ x, int/*?*/ y) =>
+    <dynamic>[x ?? (y != null ? 1 : throw 'foo'), y + 1];
+''');
+    // The null check on the RHS of the `??` doesn't promote, because it is not
+    // guaranteed to execute.
+    visitSubexpression(findNode.listLiteral('['), 'List<dynamic>',
+        nullChecked: {findNode.simple('y +')});
+  }
+
   test_binaryExpression_question_question_nullChecked() async {
     await analyze('''
 Object/*!*/ _f(int/*?*/ x, double/*?*/ y) {