Only set types of conditional expressions in strong mode.

pkg/front_end/lib/src/fasta/kernel/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.
 
 import 'package:front_end/src/fasta/kernel/kernel_shadow_ast.dart'
     show
         KernelArguments,
@@ skipping 330 matching lines @@
     var write = new PropertySet(receiverAccess(), name, value, setter)
       ..fileOffset = offsetForToken(token);
     complexAssignment?.write = write;
     return write;
   }
 
   Expression _finish(
       Expression body, KernelComplexAssignment complexAssignment) {
     var offset = offsetForToken(token);
     var nullAwareGuard = new ConditionalExpression(
-        buildIsNull(receiverAccess(), offset),
-        new NullLiteral(),
-        body,
-        const DynamicType())
+        buildIsNull(receiverAccess(), offset), new NullLiteral(), body, null)
       ..fileOffset = offset;
     if (complexAssignment != null) {
       body = makeLet(receiver, nullAwareGuard);
       KernelPropertyAssign kernelPropertyAssign = complexAssignment;
       kernelPropertyAssign.nullAwareGuard = nullAwareGuard;
       kernelPropertyAssign.desugared = body;
       return kernelPropertyAssign;
     } else {
       return new KernelNullAwarePropertyGet(receiver, nullAwareGuard)
         ..fileOffset = offset;
@@ skipping 361 matching lines @@
 
 Expression buildIsNull(Expression value, int offset) {
   return makeBinary(value, equalsName, null, new NullLiteral(), offset: offset);
 }
 
 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);
 }