Fix bug in SsaConstructionFieldTypes: handle mulitple assignments to the same field.

sdk/lib/_internal/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 1451 matching lines @@
 
   visitFieldSet(HFieldSet node) {
     Element field = node.element;
     HInstruction value = node.value;
     HType type = value.instructionType;
     // [HFieldSet] is also used for variables in try/catch.
     if (field.isField()) allSetters.add(field);
     // Don't handle fields defined in superclasses. Given that the field is
     // always added to the [allSetters] set, setting a field defined in a
     // superclass will get an inferred type of UNKNOWN.
-    if (identical(work.element.getEnclosingClass(), field.getEnclosingClass()) &&
-        !value.instructionType.isUnknown()) {
+    if (work.element.getEnclosingClass() == field.getEnclosingClass()) {
+      HType existing = currentFieldSetters[field];
+      if (existing != null) {
+        type = existing.union(type, backend.compiler);
+      }
       currentFieldSetters[field] = type;
     }
   }
 
   visitExit(HExit node) {
     // If this has been exposed then we cannot say anything about types after
     // construction.
     if (!thisExposed) {
       // Register the known field types.
       currentFieldSetters.forEach((Element element, HType type) {
+        if (type.isUnknown()) return;
         backend.registerFieldConstructor(element, type);
         allSetters.remove(element);
       });
     }
 
     // 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);
     });
   }
 }