Fix issue 6452: do not change a dynamic setter call to a field set in case the field is not assigna…

lib/compiler/implementation/ssa/optimize.dart

 // Copyright (c) 2012, 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.
 
 part of ssa;
 
 abstract class OptimizationPhase {
   String get name;
   void visitGraph(HGraph graph);
 }
@@ skipping 598 matching lines @@
       result.guaranteedType = type;
       backend.registerFieldTypesOptimization(
           work.element, field, result.guaranteedType);
     }
     return result;
   }
 
   HInstruction visitInvokeDynamicSetter(HInvokeDynamicSetter node) {
     Element field =
         findConcreteFieldForDynamicAccess(node.receiver, node.selector);
-    if (field == null) return node;
+    if (field == null || !field.isAssignable()) return node;
     HInstruction value = node.inputs[1];
     if (compiler.enableTypeAssertions) {
       HInstruction other = value.convertType(
           compiler, field, HTypeConversion.CHECKED_MODE_CHECK);
       if (other != value) {
         node.block.addBefore(node, other);
         value = other;
       }
     }
     return new HFieldSet(field, node.inputs[0], value);
@@ skipping 710 matching lines @@
     }
 
     // For other fields having setters in the generative constructor body, set
     // the type to UNKNOWN to avoid relying on the type set in the initializer
     // list.
     allSetters.forEach((Element element) {
       backend.registerFieldConstructor(element, HType.UNKNOWN);
     });
   }
 }