Avoid exceptions in the console with ill-formed flate streams

It fixes mozilla#18876.

src/core/flate_stream.js

@@ -161,8 +161,19 @@ class FlateStream extends DecodeStream {
     try {
       const { readable, writable } = new DecompressionStream("deflate");
       const writer = writable.getWriter();
-      writer.write(bytes);
-      writer.close();
+      await writer.ready;
+
+      // We can't await writer.write() because it'll block until the reader
+      // starts which happens few lines below.
+      writer
+        .write(bytes)
+        .then(async () => {
+          try {
+            await writer.ready;
+            await writer.close();
+          } catch {}
+        })
+        .catch(() => {});
 
       const chunks = [];
       let totalLength = 0;