Add a strong mode implementation of LUB to fasta; use it to infer ?:

pkg/front_end/testcases/inference/conditional_lub.dart

Index: pkg/front_end/testcases/inference/conditional_lub.dart
diff --git a/pkg/front_end/testcases/inference/conditional_lub.dart b/pkg/front_end/testcases/inference/conditional_lub.dart
new file mode 100644
index 0000000000000000000000000000000000000000..cab13164562a931326ea5d32c6c08aeb57f85425
--- /dev/null
+++ b/pkg/front_end/testcases/inference/conditional_lub.dart
@@ -0,0 +1,15 @@
+// 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;
+
+bool b = true;
+int x = 0;
+double y = 0.0;
+var /*@topType=num*/ z = b ? x : y;

[ahe, 2017/05/02 18:32:57]

It's getting closer and closer to rocket science now :-)

+
+main() {
+  var /*@type=num*/ z = b ? x : y;
+}