[ddc] Fix return type to avoid cast to bool

Now the call in `dtest()` (directly above) is known to never return
so the type of `obj` can be promoted and there is no cast in the
return statement.

Change-Id: I9096dab87cfba5ea2289530070d0214974379ebc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/155068
Reviewed-by: Mark Zhou <markzipan@google.com>
Commit-Queue: Nicholas Shahan <nshahan@google.com>

sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/operations.dart

@@ -456,7 +456,7 @@ bool dtest(obj) {
   return obj;
 }
 
-void booleanConversionFailed(obj) {
+Never booleanConversionFailed(obj) {
   var actual = typeName(getReifiedType(obj));
   throw TypeErrorImpl("type '$actual' is not a 'bool' in boolean expression");
 }