Only set types of conditional expressions in strong mode.

pkg/front_end/testcases/rasta/bad_default_constructor.dart.direct.expect

 library;
 import self as self;
+import "dart:core" as core;
 
-static method main() → dynamic
-  throw "pkg/front_end/testcases/rasta/bad_default_constructor.dart: Error: A has no constructor that takes no arguments.";
+class A extends core::Object {
+  constructor •(dynamic x) → dynamic
+    : super core::Object::•()
+    ;
+}
+class B extends self::A {
+  constructor •() → self::B
+    : invalid-initializer;
+}
+static method main() → dynamic {
+  new self::B::•();
+  invalid-expression;
+}