Remove TODOs from KernelConditionalExpression._inferExpression.

pkg/front_end/testcases/inference/conditional_upwards_inference.dart

+// Copyright (c) 2017, 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.
+
+/*@testedFeatures=inference*/
+library test;
+
+class C<T> {
+  C(List<T> x);
+}
+
+main() {
+  bool b = false;
+  List<int> l1 = /*@typeArgs=int*/ [1];
+  List<int> l2 = /*@typeArgs=int*/ [2];
+  var /*@type=C<int>*/ x = new /*@typeArgs=int*/ C(l1);
+  var /*@type=C<int>*/ y = new /*@typeArgs=int*/ C(l2);
+  var /*@type=C<int>*/ z = new /*@typeArgs=int*/ C(b ? l1 : l2);
+}