Always store the type of a conditional expression on the node.

lib/frontend/accessors.dart

 // Copyright (c) 2016, 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.
 
 /// A library to help transform compounds and null-aware accessors into
 /// let expressions.
 library kernel.frontend.accessors;
 
 import '../ast.dart';
 
 abstract class Accessor {
   Expression buildSimpleRead() {
     return _finish(_makeSimpleRead());
   }
 
   /// Returns an assignment to the accessor.
   ///
   /// The returned expression evaluates to the assigned value, unless
   /// [voidContext] is true, in which case it may evaluate to anything.
   Expression buildAssignment(Expression value, {bool voidContext: false}) {
     return _finish(_makeSimpleWrite(value, voidContext));
   }
 
-  Expression buildNullAwareAssignment(Expression value,
+  Expression buildNullAwareAssignment(Expression value, DartType type,
       {bool voidContext: false}) {
     if (voidContext) {
       return _finish(new ConditionalExpression(buildIsNull(_makeRead()),
-          _makeWrite(value, voidContext), new NullLiteral()));
+          _makeWrite(value, voidContext), new NullLiteral(), type));
     }
     var tmp = new VariableDeclaration.forValue(_makeRead());
     return _finish(makeLet(
         tmp,
         new ConditionalExpression(buildIsNull(new VariableGet(tmp)),
-            _makeWrite(value, voidContext), new VariableGet(tmp))));
+            _makeWrite(value, voidContext), new VariableGet(tmp), type)));
   }
 
   Expression buildCompoundAssignment(Name binaryOperator, Expression value,
       {bool voidContext: false, Procedure interfaceTarget}) {
     return _finish(_makeWrite(
         makeBinary(_makeRead(), binaryOperator, interfaceTarget, value),
         voidContext));
   }
 
   Expression buildPrefixIncrement(Name binaryOperator,
@@ skipping 98 matching lines @@
 
   _makeWrite(Expression value, bool voidContext) {
     return new PropertySet(new ThisExpression(), name, value, setter);
   }
 }
 
 class NullAwarePropertyAccessor extends Accessor {
   VariableDeclaration receiver;
   Name name;
   Member getter, setter;
+  DartType type;
 
   NullAwarePropertyAccessor(
-      Expression receiver, this.name, this.getter, this.setter)
+      Expression receiver, this.name, this.getter, this.setter, this.type)
       : this.receiver = makeOrReuseVariable(receiver);
 
   receiverAccess() => new VariableGet(receiver);
 
   _makeRead() => new PropertyGet(receiverAccess(), name, getter);
 
   _makeWrite(Expression value, bool voidContext) {
     return new PropertySet(receiverAccess(), name, value, setter);
   }
 
   _finish(Expression body) => makeLet(
       receiver,
       new ConditionalExpression(
-          buildIsNull(receiverAccess()), new NullLiteral(), body));
+          buildIsNull(receiverAccess()), new NullLiteral(), body, type));
 }
 
 class SuperPropertyAccessor extends Accessor {
   Name name;
   Member getter, setter;
 
   SuperPropertyAccessor(this.name, this.getter, this.setter);
 
   _makeRead() => new SuperPropertyGet(name, getter);
 
@@ skipping 224 matching lines @@
 
 VariableDeclaration makeOrReuseVariable(Expression value) {
   // TODO: Devise a way to remember if a variable declaration was reused
   // or is fresh (hence needs a let binding).
   return new VariableDeclaration.forValue(value);
 }
 
 Expression wrapInvalid(Expression e) {
   return new Let(new VariableDeclaration.forValue(e), new InvalidExpression());
 }