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

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

Index: pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
index c61fe0bb5a2db33dab7c98bd626c7711a8b17f38..6737745c51dca31d17e7aa2a8df78545dccfe054 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
@@ -35,6 +35,21 @@ class KernelBlock extends Block implements KernelStatement {
   }
 }
 
+/// Concrete shadow object representing a conditional expression in kernel form.
+class KernelConditionalExpression extends ConditionalExpression
+    implements KernelExpression {
+  KernelConditionalExpression(
+      Expression condition, Expression then, Expression otherwise)
+      : super(condition, then, otherwise, const DynamicType());
+
+  @override
+  DartType _inferExpression(
+      KernelTypeInferrer inferrer, DartType typeContext, bool typeNeeded) {
+    return inferrer.inferConditionalExpression(typeContext, typeNeeded,
+        condition, then, otherwise, (type) => staticType = type);
+  }
+}
+
 /// Common base class for shadow objects representing expressions in kernel
 /// form.
 abstract class KernelExpression implements Expression {