Issue 41019. Fix for readType of IndexExpression.

Bug: https://github.com/dart-lang/sdk/issues/41019
Change-Id: I861b355c14c6bb30e570bb0c0101871db423ffb0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/139821
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

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

@@ -204,9 +204,7 @@ class PostfixExpressionResolver {
     operand.accept(_resolver);
     operand = node.operand;
 
-    var operandType = getReadType(
-      operand,
-    );
+    var operandType = getReadType(operand);
 
     var type = _typeSystem.promoteToNonNull(operandType);
     _inferenceHelper.recordStaticType(node, type);

pkg/analyzer/lib/src/task/strong/checker.dart

@@ -37,10 +37,14 @@ DartType getExpressionType(
 
 DartType getReadType(Expression expression) {
   if (expression is IndexExpression) {
-    var staticElement = expression.auxiliaryElements?.staticElement;
-    return staticElement == null
-        ? DynamicTypeImpl.instance
-        : staticElement.returnType;
+    var aux = expression.auxiliaryElements;
+    if (aux != null) {
+      var staticElement = aux.staticElement;
+      return staticElement == null
+          ? DynamicTypeImpl.instance
+          : staticElement.returnType;
+    }
+    return expression.staticType;
   }
   {
     Element setter;

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

@@ -159,6 +159,31 @@ main(Function f2) {
 ''');
   }
 
+  test_nullCheck_indexExpression() async {
+    await assertNoErrorsInCode(r'''
+main(Map<String, int> a) {
+  int v = a['foo']!;
+  v;
+}
+''');
+
+    assertIndexExpression(
+      findNode.index('a['),
+      readElement: elementMatcher(
+        mapElement.getMethod('[]'),
+        substitution: {'K': 'String', 'V': 'int'},
+      ),
+      writeElement: null,
+      type: 'int?',
+    );
+
+    assertPostfixExpression(
+      findNode.postfix(']!'),
+      element: null,
+      type: 'int',
+    );
+  }
+
   test_nullCheck_null() async {
     await assertNoErrorsInCode('''
 main(Null x) {