Rationalize coercions

lib/src/checker/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.
 
 library dev_compiler.src.checker.checker;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/scanner.dart' show Token, TokenType;
@@ skipping 735 matching lines @@
 
       if (!_rules.isSubTypeOf(returnType, lhsType)) {
         final numType = _rules.provider.numType;
         // 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.
           staticInfo = DownCast.create(
               _rules, expr.rightHandSide, Coercion.cast(rhsType, lhsType));
-          expr.rightHandSide = staticInfo;
+          if (staticInfo is DownCast) {
+            expr.rightHandSide = staticInfo;
+          }
           rhsType = lhsType;
         } else {
           // Static type error
           staticInfo = new StaticTypeError(_rules, expr, lhsType);
         }
         _recordMessage(staticInfo);
       }
 
       // Check the rhs type
       if (staticInfo is! Conversion) {
         var paramType = paramTypes.first;
         staticInfo = _rules.checkAssignment(
             expr.rightHandSide, paramType, _constantContext);
         _recordMessage(staticInfo);
         if (staticInfo is Conversion) expr.rightHandSide = staticInfo;
       }
     }
   }
 
   void _recordDynamicInvoke(AstNode node) {
     _reporter.log(new DynamicInvoke(_rules, node));
   }
 
   void _recordMessage(StaticInfo info) {
     if (info == null) return;
     if (info.level >= logger.Level.SEVERE) _failure = true;
     _reporter.log(info);
   }
 }