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

pkg/front_end/lib/src/fasta/kernel/body_builder.dart

Index: pkg/front_end/lib/src/fasta/kernel/body_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/body_builder.dart b/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
index 9336bee1aedefc7717232391b9f1e326535cf1d8..c43ba44de6c4d7cd1c3b5b12b36e5134bcc70fa4 100644
--- a/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
@@ -1505,8 +1505,8 @@ class BodyBuilder extends ScopeListener<JumpTarget> implements BuilderHelper {
     Expression elseExpression = popForValue();
     Expression thenExpression = popForValue();
     Expression condition = popForValue();
-    push(new ConditionalExpression(
-        condition, thenExpression, elseExpression, const DynamicType()));
+    push(astFactory.conditionalExpression(
+        condition, thenExpression, elseExpression));
   }
 
   @override