[html] Reset of ScriptElement.src prior to async execution has no effect

[DartBot]

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

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The test fails

co19/src/LayoutTests/fast/dom/HTMLScriptElement/remove-source_t01.dart

src of a script is reset after insertion of script into document head

    var script = document.createElement("script");
    script.src = "data:text/javascript,debug('FAIL');";
    document.head.append(script);
    script.src = "";

but the old script still executes

[kevmoo]

Added Library-Html, Triaged labels.

[kasperl]

Added Area-Library label.

[alan-knight]

Removed Priority-Unassigned label.
Added Priority-Medium label.