Reduce usage of buildUnsupported.

test/kernel/regression/bad_default_constructor.dart.txt

 library;
 import self as self;
 import "dart:core" as core;
 
 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::•();
-  throw "Kernel IR not yet implemented for 'handleError'.";
+  invalid-expression;
 }