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

pkg/front_end/lib/src/fasta/builder/ast_factory.dart

Index: pkg/front_end/lib/src/fasta/builder/ast_factory.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/ast_factory.dart b/pkg/front_end/lib/src/fasta/builder/ast_factory.dart
index bdf5e5c0b9b05f3260c975b6ffaac54009eff4af..2d15dabf121bc8109eb8160e248d5fbb042c723d 100644
--- a/pkg/front_end/lib/src/fasta/builder/ast_factory.dart
+++ b/pkg/front_end/lib/src/fasta/builder/ast_factory.dart
@@ -38,6 +38,10 @@ abstract class AstFactory<V> {
   /// Creates a statement block.
   Block block(List<Statement> statements, Token beginToken);
 
+  /// Creates a conditional expression.
+  ConditionalExpression conditionalExpression(Expression condition,
+      Expression thenExpression, Expression elseExpression);
+
   /// Creates an expression statement.
   ExpressionStatement expressionStatement(Expression expression);