Fix assertion when writing to a closed writable stream

Closes #1341.

Author: domenic

index.bs

@@ -5401,8 +5401,8 @@ The following abstract operations support the implementation of the
  |chunk|)</dfn> performs the following steps:
 
  1. If |controller|.[=WritableStreamDefaultController/[[strategySizeAlgorithm]]=] is undefined, then:
-  1. Assert: |controller|.[=WritableStreamDefaultController/[[stream]]=].[=WritableStream/[[state]]=] is "`erroring`" or
-     "`errored`".
+  1. Assert: |controller|.[=WritableStreamDefaultController/[[stream]]=].[=WritableStream/[[state]]=] is not
+     "`writable`".
   1. Return 1.
  1. Let |returnValue| be the result of performing
     |controller|.[=WritableStreamDefaultController/[[strategySizeAlgorithm]]=], passing in |chunk|,

reference-implementation/lib/abstract-ops/writable-streams.js

@@ -663,7 +663,7 @@ function WritableStreamDefaultControllerGetBackpressure(controller) {
 
 function WritableStreamDefaultControllerGetChunkSize(controller, chunk) {
   if (controller._strategySizeAlgorithm === undefined) {
-    assert(controller._stream._state === 'erroring' || controller._stream._state === 'errored');
+    assert(controller._stream._state !== 'writable');
     return 1;
   }