fix #26912, correct handling of op assign

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

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.src.generated.error_verifier;
 
 import 'dart:collection';
 import "dart:math" as math;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 4047 matching lines @@
    * that the given assignment is valid. The [lhs] is the left hand side
    * expression. The [rhs] is the right hand side expression.
    *
    * See [StaticTypeWarningCode.INVALID_ASSIGNMENT].
    */
   void _checkForInvalidCompoundAssignment(
       AssignmentExpression assignment, Expression lhs, Expression rhs) {
     if (lhs == null) {
       return;
     }
-    VariableElement leftVariableElement = getVariableElement(lhs);
-    DartType leftType = (leftVariableElement == null)
-        ? getStaticType(lhs)
-        : leftVariableElement.type;
-    MethodElement invokedMethod = assignment.staticElement;
-    if (invokedMethod == null) {
-      return;
-    }
-    DartType rightType = invokedMethod.type.returnType;
-    if (leftType == null || rightType == null) {
-      return;
-    }
+    DartType leftType = getStaticType(lhs);
+    DartType rightType = getStaticType(assignment);
     if (!_typeSystem.isAssignableTo(rightType, leftType)) {
       _errorReporter.reportTypeErrorForNode(
           StaticTypeWarningCode.INVALID_ASSIGNMENT, rhs, [rightType, leftType]);
     }
   }
 
   /**
    * Check the given [initializer] to ensure that the field being initialized is
    * a valid field. The [fieldName] is the field name from the
    * [ConstructorFieldInitializer]. The [staticElement] is the static element
@@ skipping 2370 matching lines @@
 class _InvocationCollector extends RecursiveAstVisitor {
   final List<String> superCalls = <String>[];
 
   @override
   visitMethodInvocation(MethodInvocation node) {
     if (node.target is SuperExpression) {
       superCalls.add(node.methodName.name);
     }
   }
 }