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

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

Index: pkg/front_end/lib/src/fasta/kernel/kernel_ast_factory.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_ast_factory.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_ast_factory.dart
index 555f799eff019772132cafdcf6d1bd7eb2cc41a9..3d0be627b1cb13134f0ca70902646606d6dadd1f 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_ast_factory.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_ast_factory.dart
@@ -18,6 +18,13 @@ class KernelAstFactory implements AstFactory<VariableDeclaration> {
   }
 
   @override
+  ConditionalExpression conditionalExpression(Expression condition,
+      Expression thenExpression, Expression elseExpression) {
+    return new KernelConditionalExpression(
+        condition, thenExpression, elseExpression);
+  }
+
+  @override
   ExpressionStatement expressionStatement(Expression expression) {
     return new KernelExpressionStatement(expression);
   }