[html] InputElement.stepUp(), stepDown() should throw on unsupported types

[DartBot]

This issue was originally filed by Ilya.Va...@gmail.com

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Found during port of layout tests.

import "dart:html"; import "../../testcommon.dart"; import "../../../Utils/async_utils.dart";

_shouldThrow(func())   => shouldThrow(func, (e) => e is DomException && e.name == DomException.INVALID_STATE);

main() {   var input = document.createElement('input');

  input.type = 'text';   input.step = "3";   input.min = "";   input.max = "";   input.value = "2";   _shouldThrow(() => input.stepDown()); // does not throw   _shouldThrow(() => input.stepDown(0));   _shouldThrow(() => input.stepUp()); // does not throw   _shouldThrow(() => input.stepUp(0)); }

[dgrove]

Added Library-Html, Triaged labels.

[alan-knight]

Removed Priority-Unassigned label. Added Priority-Medium, Area-Library, Accepted labels.