Kernel: Do not try to use return value of []=

pkg/kernel/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';
 
@@ skipping 14 matching lines @@
   /// 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, DartType type,
       {bool voidContext: false}) {
     if (voidContext) {
       return _finish(new ConditionalExpression(buildIsNull(_makeRead()),
-          _makeWrite(value, voidContext), new NullLiteral(), type));
+          _makeWrite(value, false), 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), type)));
+            _makeWrite(value, false), new VariableGet(tmp), type)));
   }
 
   Expression buildCompoundAssignment(Name binaryOperator, Expression value,
       {int offset: TreeNode.noOffset,
       bool voidContext: false,
       Procedure interfaceTarget}) {
     return _finish(_makeWrite(
         builtBinary = makeBinary(
             _makeRead(), binaryOperator, interfaceTarget, value,
             offset: offset),
@@ skipping 374 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());
 }