Split out common steps into a separate algorithm

fetch.bs

@@ -7399,11 +7399,8 @@ enum ResponseType { "basic", "cors", "default", "error", "opaque", "opaqueredire
  <li><p>Return <var>responseObject</var>.
 </ol>
 
-<hr>
-
-<p>The
-<dfn constructor for=Response id=dom-response lt="Response(body, init)"><code>new Response(<var>body</var>, <var>init</var>)</code></dfn>
-constructor steps are:
+<p>To <dfn>initialize a response</dfn>, given a {{Response}} <var>response</var>, and
+{{ResponseInit}} <var>init</var>, run these steps:
 
 <ol>
  <li><p>If <var>init</var>["{{ResponseInit/status}}"] is not in the range 200 to 599, inclusive,
@@ -7412,30 +7409,40 @@ constructor steps are:
  <li><p>If <var>init</var>["{{ResponseInit/statusText}}"] does not match the
  <a spec=http>reason-phrase</a> token production, then <a>throw</a> a {{TypeError}}.
 
+ <li><p>Set <var>response</var>'s <a for=Response>response</a>'s <a for=response>status</a> to
+ <var>init</var>["{{ResponseInit/status}}"].
+
+ <li><p>Set <var>response</var>'s <a for=Response>response</a>'s <a for=response>status message</a> to
+ <var>init</var>["{{ResponseInit/statusText}}"].
+
+ <li><p>If <var>init</var>["{{ResponseInit/headers}}"] <a for=map>exists</a>, then
+ <a for=Headers>fill</a> <var>response</var>'s <a for=Response>headers</a> with
+ <var>init</var>["{{ResponseInit/headers}}"].
+</ol>
+
+<hr>
+
+<p>The
+<dfn constructor for=Response id=dom-response lt="Response(body, init)"><code>new Response(<var>body</var>, <var>init</var>)</code></dfn>
+constructor steps are:
+
+<ol>
  <li><p>Set <a>this</a>'s <a for=Response>response</a> to a new <a for=/>response</a>.
 
  <li><p>Set <a>this</a>'s <a for=Response>headers</a> to a <a for=/>new</a> {{Headers}} object with
  <a>this</a>'s <a>relevant Realm</a>, whose <a for=Headers>header list</a> is <a>this</a>'s
  <a for=Response>response</a>'s <a for=response>header list</a> and <a for=Headers>guard</a> is
  "<code>response</code>".
 
- <li><p>Set <a>this</a>'s <a for=Response>response</a>'s <a for=response>status</a> to
- <var>init</var>["{{ResponseInit/status}}"].
-
- <li><p>Set <a>this</a>'s <a for=Response>response</a>'s <a for=response>status message</a> to
- <var>init</var>["{{ResponseInit/statusText}}"].
-
- <li><p>If <var>init</var>["{{ResponseInit/headers}}"] <a for=map>exists</a>, then
- <a for=Headers>fill</a> <a>this</a>'s <a for=Response>headers</a> with
- <var>init</var>["{{ResponseInit/headers}}"].
+ <li><p>Perform <a>initialize a response</a> given <a>this</a>, and <var>init</var>.
 
  <li>
   <p>If <var>body</var> is non-null, then:
 
   <ol>
    <li>
-    <p>If <var>init</var>["{{ResponseInit/status}}"] is a <a>null body status</a>, then <a>throw</a>
-    a {{TypeError}}.
+    <p>If <a>this</a>'s <a for=Response>response</a>'s <a for=response>status</a> is a
+    <a>null body status</a>, then <a>throw</a> a {{TypeError}}.
 
     <p class="note no-backref">101 is included in <a>null body status</a> due to its use elsewhere.
     It does not affect this step.
@@ -7491,31 +7498,17 @@ are:
 are:
 
 <ol>
- <li><p>If <var>init</var>["{{ResponseInit/status}}"] is not in the range 200 to 599, inclusive,
- then <a>throw</a> a {{RangeError}}.
-
- <li><p>If <var>init</var>["{{ResponseInit/status}}"] is a <a>null body status</a>, then
- <a>throw</a> a {{TypeError}}.
-
- <li><p>If <var>init</var>["{{ResponseInit/statusText}}"] does not match the
- <a spec=http>reason-phrase</a> token production, then <a>throw</a> a {{TypeError}}.
-
  <li><p>Let <var>responseObject</var> be the result of <a for=Response>creating</a> a {{Response}}
  object, given a new <a for=/>response</a>, "<code>response</code>", and <a>this</a>'s
  <a>relevant Realm</a>.
 
- <li><p>Set <var>responseObject</var>'s <a for=Response>response</a>'s <a for=response>status</a> to
- <var>init</var>["{{ResponseInit/status}}"].
-
- <li><p>Set <var>responseObject</var>'s <a for=Response>response</a>'s
- <a for=response>status message</a> to <var>init</var>["{{ResponseInit/statusText}}"].
+ <li><p>Perform <a>initialize a response</a> given <var>responseObject</var>, and <var>init</var>.
 
- <li><p>If <var>init</var>["{{ResponseInit/headers}}"] <a for=map>exists</a>, then
- <a for=Headers>fill</a> <var>responseObject</var>'s <a for=Response>headers</a> with
- <var>init</var>["{{ResponseInit/headers}}"].
+ <li><p>If <var>responseObject</var>'s <a for=Response>response</a>'s <a for=response>status</a> is
+ a <a>null body status</a>, then <a>throw</a> a {{TypeError}}.
 
  <li><p>Set <var>responseObject</var>'s <a for=Response>response</a>'s <a for=response>body</a> to
- the result of running <a>serialize a JavaScript value to a JSON bytes</a> on <var>data</var>.
+ the result of running <a>serialize a JavaScript value to JSON bytes</a> on <var>data</var>.
 
  <li><p>If <var>responseObject</var>'s <a for=Response>response</a>'s <a for=response>header list</a>
  <a for="header list">does not contain</a> `<code>Content-Type</code>`, then