Editorial: separate TextDecoderStream constructor to two algorithms

One (the constructor) for use by JS and one (the set up steps) for use by other specifications.

See whatwg/fetch#1862 (review).

Author: noamr

encoding.bs

@@ -1851,23 +1851,41 @@ constructor steps are:
 
  <li><p>If <var>encoding</var> is failure or <a>replacement</a>, then <a>throw</a> a {{RangeError}}.
 
- <li><p>Set <a>this</a>'s <a for=TextDecoderCommon>encoding</a> to <var>encoding</var>.
+ <li><p>Let <var>errorMode</var> be "<code>fatal</code>" if
+ <var>options</var>["{{TextDecoderOptions/fatal}}"] is true; otherwise "<code>replacement</code>".
 
- <li><p>If <var>options</var>["{{TextDecoderOptions/fatal}}"] is true, then set <a>this</a>'s
- <a for=TextDecoderCommon>error mode</a> to "<code>fatal</code>".
+ <li><p><a>Set up a text decoder stream</a> with <a>this</a>, <var>encoding</var>,
+ <var>errorMode</var>, and <var>options</var>["{{TextDecoderOptions/ignoreBOM}}"].
+</ol>
+</div>
 
- <li><p>Set <a>this</a>'s <a for=TextDecoderCommon>ignore BOM</a> to
- <var>options</var>["{{TextDecoderOptions/ignoreBOM}}"].
+<div algorithm>
+<p>To <dfn export>set up a text decoder stream</dfn> given a {{TextDecoderStream}} object
+<var>stream</var>, an optional [=/encoding=] <var>encoding</var> (default <a>UTF-8</a>), an optional
+<a for=/>error mode</a> <var>errorMode</var> (default "<code>replacement</code>"), and an optional
+boolean <var>ignoreBOM</var> (default false), run these steps:
 
- <li><p>Set <a>this</a>'s <a for=TextDecoderCommon>decoder</a> to a new instance of <a>this</a>'s
- <a for=TextDecoderCommon>encoding</a>'s <a for=/>decoder</a>, and set <a>this</a>'s
- <a for=TextDecoderCommon>I/O queue</a> to a new <a for=/>I/O queue</a>.
+<ol>
+ <li><p><a>Assert</a>: <var>encoding</var> is not <a>replacement</a>.
+
+ <li><p>Set <var>stream</var>'s <a for=TextDecoderCommon>encoding</a> to <var>encoding</var>.
+
+ <li><p>Set <var>stream</var>'s <a for=TextDecoderCommon>error mode</a> to <var>errorMode</var>.
+
+ <li><p>Set <var>stream</var>'s <a for=TextDecoderCommon>ignore BOM</a> to <var>ignoreBOM</var>.
+
+ <li><p>Set <var>stream</var>'s <a for=TextDecoderCommon>decoder</a> to a new instance of
+ <var>encoding</var>'s <a for=/>decoder</a>.
+
+ <li><p>Set <var>stream</var>'s <a for=TextDecoderCommon>I/O queue</a> to a new <a for=/>I/O
+ queue</a>.
 
  <li><p>Let <var>transformAlgorithm</var> be an algorithm which takes a <var>chunk</var> argument
- and runs the <a>decode and enqueue a chunk</a> algorithm with <a>this</a> and <var>chunk</var>.
+ and runs the <a>decode and enqueue a chunk</a> algorithm with <var>stream</var> and
+ <var>chunk</var>.
 
- <li><p>Let <var>flushAlgorithm</var> be an algorithm which takes no arguments and runs the
- <a>flush and enqueue</a> algorithm with <a>this</a>.
+ <li><p>Let <var>flushAlgorithm</var> be an algorithm which takes no arguments and runs the <a>flush
+ and enqueue</a> algorithm with <var>stream</var>.
 
  <li><p>Let <var>transformStream</var> be a [=new=] {{TransformStream}}.
 
@@ -1877,7 +1895,8 @@ constructor steps are:
  <a for="TransformStream/set up"><var ignore>flushAlgorithm</var></a> set to
  <var>flushAlgorithm</var>.
 
- <li><p>Set <a>this</a>'s <a for=GenericTransformStream>transform</a> to <var>transformStream</var>.
+ <li><p>Set <var>stream</var>'s <a for=GenericTransformStream>transform</a> to
+ <var>transformStream</var>.
 </ol>
 </div>