fix #26912, correct handling of op assign

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

 // Copyright (c) 2015, 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.
 
 // TODO(jmesserly): this was ported from package:dev_compiler, and needs to be
 // refactored to fit into analyzer.
 library analyzer.src.task.strong.checker;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 615 matching lines @@
       assert(functionType.optionalParameterTypes.isEmpty);
 
       // Check the LHS type.
       var rhsType = _getStaticType(expr.rightHandSide);
       var lhsType = _getStaticType(expr.leftHandSide);
       var returnType = rules.refineBinaryExpressionType(
           typeProvider, lhsType, op, rhsType, functionType.returnType);
 
       if (!rules.isSubtypeOf(returnType, lhsType)) {
         final numType = typeProvider.numType;
+        // TODO(jmesserly): this seems to duplicate logic in StaticTypeAnalyzer.
         // Try to fix up the numerical case if possible.
         if (rules.isSubtypeOf(lhsType, numType) &&
             rules.isSubtypeOf(lhsType, rhsType)) {
           // This is also slightly different from spec, but allows us to keep
           // compound operators in the int += num and num += dynamic cases.
           _recordImplicitCast(expr.rightHandSide, rhsType, lhsType);
         } else {
+          // TODO(jmesserly): this results in a duplicate error, because
+          // ErrorVerifier also reports it.
           _recordMessage(expr, StrongModeCode.STATIC_TYPE_ERROR,
               [expr, returnType, lhsType]);
         }
       } else {
         // Check the RHS type.
         //
         // This is only needed if we didn't already need a cast, and avoids
         // emitting two messages for the same expression.
         _checkDowncast(expr.rightHandSide, paramTypes.first);
       }
@@ skipping 714 matching lines @@
     var visited = new Set<InterfaceType>();
     do {
       visited.add(current);
       current.mixins.reversed.forEach(
           (m) => _checkIndividualOverridesFromClass(node, m, seen, true));
       _checkIndividualOverridesFromClass(node, current.superclass, seen, true);
       current = current.superclass;
     } while (!current.isObject && !visited.contains(current));
   }
 }