Add "abort when"/"if aborted" construct for Fetch and elsewhere

Fixes #143.

infra.bs

@@ -175,6 +175,78 @@ caller's algorithm. Using prose the caller has the ability to "catch" the except
 another action.
 
 
+<h3 id=algorithm-conditional-abort>Conditional abort</h3>
+
+<p>Sometimes it is useful to stop performing a series of steps once a condition becomes true.
+
+<p>To do this, state that a given series of steps will <dfn export>abort when</dfn> a specific
+<var>condition</var> is reached. This indicates that the specified steps must be evaluated, not
+as-written, but by additionally inserting a step before each of them that evaluates
+<var>condition</var>, and if <var>condition</var> evaluates to true, skips the remaining steps.
+
+<p>In such algorithms, the subsequent step can be annotated to run <dfn export>if aborted</dfn>, in
+which case it must run if any of the preceding steps were skipped due to the <var>condition</var>
+of the preceding <a>abort when</a> step evaluated to true.
+
+<div class=example id=example-conditional-abort>
+ <p>The following algorithm
+
+ <ol>
+  <li><p>Let |result| be an empty <a>list</a>.
+
+  <li>
+   <p>Run these steps, but <a>abort when</a> the user clicks the "Cancel" button:
+
+   <ol>
+    <li><p>Compute the first million digits of <var>π</var>, and <a for=list>append</a> the result
+    to |result|.
+
+    <li><p>Compute the first million digits of |e|, and <a for=list>append</a> the result to
+    |result|.
+
+    <li><p>Compute the first million digits of <var>φ</var>, and <a for=list>append</a> the result
+    to |result|.
+   </ol>
+  </li>
+
+  <li><p><a>If aborted</a>, <a for=list>append</a> "<code>Didn't finish!</code>" to |result|.
+ </ol>
+
+ <p>is equivalent to the more verbose formulation</p>
+
+ <ol>
+  <li><p>Let |result| be an empty <a>list</a>.
+
+  <li>
+   <p>If the user has not clicked the "Cancel" button, then:
+
+   <ol>
+    <li><p>Compute the first million digits of <var>π</var>, and <a for=list>append</a> the result
+    to |result|.
+
+    <li>
+     <p>If the user has not clicked the "Cancel" button, then:
+
+     <ol>
+      <li><p>Compute the first million digits of |e|, and <a for=list>append</a> the result to
+      |result|.
+
+      <li><p>If the user has not clicked the "Cancel" button, then compute the first million digits
+      of <var>φ</var>, and <a for=list>append</a> the result to |result|.
+     </ol>
+   </ol>
+
+  <li><p>If the user clicked the "Cancel" button, then <a for=list>append</a>
+  "<code>Didn't finish!</code>" to |result|.
+ </ol>
+</div>
+
+<p class=note>Whenever this construct is used, implementations are allowed to evaluate
+<var>condition</var> during the specified steps rather than before and after each step, as long as
+the end result is indistinguishable. For instance, as long as |result| in the above example is not
+mutated during a compute operation, the user agent could stop the computation.
+
+
 <h3 id=algorithm-iteration>Iteration</h3>
 
 <p>There's a variety of ways to repeat a set of steps until a condition is reached.