Do not let mutation observer callbacks influence the used signal slot…

… list

Fixes WICG/webcomponents#518.

dom.bs

@@ -2903,6 +2903,11 @@ To <dfn export>notify mutation observers</dfn>, run these steps:
  <a>unit of related similar-origin browsing contexts</a>'
  list of {{MutationObserver}} objects.
 
+ <li><p>Let <var>signalList</var> be a copy of
+ <a>unit of related similar-origin browsing contexts</a>' <a>signal slot list</a>.
+
+ <li><p>Empty <a>unit of related similar-origin browsing contexts</a>' <a>signal slot list</a>.
+
  <li>
   For each {{MutationObserver}} object <var>mo</var> in <var>notify list</var>,
   <a>execute a compound microtask subtask</a> to run these steps: [[!HTML]]
@@ -2923,11 +2928,6 @@ To <dfn export>notify mutation observers</dfn>, run these steps:
    <a>report the exception</a>.
   </ol>
 
- <li><p>Let <var>signalList</var> be a copy of
- <a>unit of related similar-origin browsing contexts</a>' <a>signal slot list</a>.
-
- <li><p>Empty <a>unit of related similar-origin browsing contexts</a>' <a>signal slot list</a>.
-
  <li><p>For each <a>slot</a> <var>slot</var> in <var>signalList</var>, in order,
  <a>fire an event</a> named <code>slotchange</code>, with its {{Event/bubbles}} attribute set to
  true, at <var>slot</var>.

dom.html

@@ -72,7 +72,7 @@
    <p data-fill-with="logo"><a class="logo" href="https://whatwg.org/"> <img alt="WHATWG" height="100" src="https://resources.whatwg.org/logo-dom.svg"> </a> </p>
    <hgroup>
     <h1 class="p-name no-ref allcaps" id="title">DOM</h1>
-    <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Living Standard — Last Updated <time class="dt-updated" datetime="2016-06-08">8 June 2016</time></span></h2>
+    <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Living Standard — Last Updated <time class="dt-updated" datetime="2016-06-09">9 June 2016</time></span></h2>
    </hgroup>
    <div data-fill-with="spec-metadata">
     <dl>
@@ -1818,6 +1818,10 @@ <h3 class="heading settled" data-level="4.3" id="mutation-observers"><span class
     <li>Unset <a data-link-type="dfn" href="#mutation-observer-compound-microtask-queued-flag">mutation observer compound microtask queued flag</a>. 
     <li>Let <var>notify list</var> be a copy of <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/browsers.html#unit-of-related-similar-origin-browsing-contexts">unit of related similar-origin browsing contexts</a>'
  list of <code class="idl"><a data-link-type="idl" href="#mutationobserver">MutationObserver</a></code> objects. 
+    <li>
+     <p>Let <var>signalList</var> be a copy of <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/browsers.html#unit-of-related-similar-origin-browsing-contexts">unit of related similar-origin browsing contexts</a>' <a data-link-type="dfn" href="#signal-slot-list">signal slot list</a>. </p>
+    <li>
+     <p>Empty <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/browsers.html#unit-of-related-similar-origin-browsing-contexts">unit of related similar-origin browsing contexts</a>' <a data-link-type="dfn" href="#signal-slot-list">signal slot list</a>. </p>
     <li>
       For each <code class="idl"><a data-link-type="idl" href="#mutationobserver">MutationObserver</a></code> object <var>mo</var> in <var>notify list</var>, <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/webappapis.html#execute-a-compound-microtask-subtask">execute a compound microtask subtask</a> to run these steps: <a data-link-type="biblio" href="#biblio-html">[HTML]</a> 
      <ol>
@@ -1826,10 +1830,6 @@ <h3 class="heading settled" data-level="4.3" id="mutation-observers"><span class
       <li>Remove all <a data-link-type="dfn" href="#transient-registered-observer">transient registered observers</a> whose <b>observer</b> is <var>mo</var>. 
       <li>If <var>queue</var> is non-empty, call <var>mo</var>’s <a data-link-type="dfn" href="#concept-mo-callback">callback</a> with <var>queue</var> as first argument, and <var>mo</var> (itself) as second argument and <a data-link-type="dfn" href="https://heycam.github.io/webidl/#dfn-callback-this-value">callback this value</a>. If this throws an exception, <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/webappapis.html#report-the-exception">report the exception</a>. 
      </ol>
-    <li>
-     <p>Let <var>signalList</var> be a copy of <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/browsers.html#unit-of-related-similar-origin-browsing-contexts">unit of related similar-origin browsing contexts</a>' <a data-link-type="dfn" href="#signal-slot-list">signal slot list</a>. </p>
-    <li>
-     <p>Empty <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/browsers.html#unit-of-related-similar-origin-browsing-contexts">unit of related similar-origin browsing contexts</a>' <a data-link-type="dfn" href="#signal-slot-list">signal slot list</a>. </p>
     <li>
      <p>For each <a data-link-type="dfn" href="#concept-slot">slot</a> <var>slot</var> in <var>signalList</var>, in order, <a data-link-type="dfn" href="#concept-event-fire">fire an event</a> named <code>slotchange</code>, with its <code class="idl"><a data-link-type="idl" href="#dom-event-bubbles">bubbles</a></code> attribute set to
  true, at <var>slot</var>. </p>