Add some extra asserts to TransformStream to clarify invariants

It's a subtle point that TransformStreamEnqueueToReadable only ever
changes _backpressure to true, so add an assert for it.

The fact that WritableStream only ever calls sink write() when the state
is "writable" is important to the correct functioning of TransformStream,
so add an assert for that too. Also assert that
_backpressureChangePromise has been initialised in write(), for
consistency with pull().

Also a minor comment fix to TransformStreamError.

No functional changes.

reference-implementation/lib/transform-stream.js

@@ -118,7 +118,8 @@ function TransformStreamEnqueueToReadable(stream, chunk) {
 
   const backpressure = ReadableStreamDefaultControllerHasBackpressure(controller);
   if (backpressure !== stream._backpressure) {
-    TransformStreamSetBackpressure(stream, backpressure);
+    assert(backpressure === true, 'backpressure is *true*');
+    TransformStreamSetBackpressure(stream, true);
   }
 }
 
@@ -131,9 +132,9 @@ function TransformStreamError(stream, e) {
     ReadableStreamDefaultControllerError(stream._readable._readableStreamController, e);
   }
   if (stream._backpressure === true) {
-    // Pretend that pull() was called to permit any pending write() or start() calls to complete.
-    // TransformStreamSetBackpressure() cannot be called from enqueue() or pull() once the ReadableStream is errored,
-    // so this will will be the final time _backpressure is set.
+    // Pretend that pull() was called to permit any pending write() calls to complete. TransformStreamSetBackpressure()
+    // cannot be called from enqueue() or pull() once the ReadableStream is errored, so this will will be the final time
+    // _backpressure is set.
     TransformStreamSetBackpressure(stream, false);
   }
 }
@@ -277,8 +278,11 @@ class TransformStreamDefaultSink {
     // console.log('TransformStreamDefaultSink.write()');
 
     const stream = this._ownerTransformStream;
+    assert(stream._writable._state === 'writable', 'stream.[[writable]].[[state]] is `"writable"`');
 
     if (stream._backpressure === true) {
+      assert(stream._backpressureChangePromise !== undefined,
+             '_backpressureChangePromise should have been initialized');
       return stream._backpressureChangePromise
           .then(() => {
             const writable = stream._writable;