[vm] Fix Google3 ClangTidy warnings.

TEST=presubmit
Change-Id: Ifa4e77cc4548729526a90683ee163c68517e87ef
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/433001
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

runtime/vm/dart_api_impl.cc

@@ -3404,9 +3404,7 @@ static ObjectPtr ThrowArgumentError(const char* exception_message) {
     saved_exception = &Instance::Handle(raw_exception);
   }
   Exceptions::Throw(thread, *saved_exception);
-  const String& message =
-      String::Handle(String::New("Exception was not thrown, internal error"));
-  return ApiError::New(message);
+  UNREACHABLE();
 }
 
 // TODO(sgjesse): value should always be smaller then 0xff. Add error handling.