extracted blobSlicer, fixed download cancel tests

app/blobSlicer.js

@@ -0,0 +1,27 @@
+export default class BlobSlicer {
+  constructor(blob, size) {
+    this.blob = blob;
+    this.index = 0;
+    this.chunkSize = size;
+  }
+
+  pull(controller) {
+    return new Promise((resolve, reject) => {
+      const bytesLeft = this.blob.size - this.index;
+      if (bytesLeft <= 0) {
+        controller.close();
+        return resolve();
+      }
+      const size = Math.min(this.chunkSize, bytesLeft);
+      const blob = this.blob.slice(this.index, this.index + size);
+      const reader = new FileReader();
+      reader.onload = () => {
+        controller.enqueue(new Uint8Array(reader.result));
+        resolve();
+      };
+      reader.onerror = reject;
+      reader.readAsArrayBuffer(blob);
+      this.index += size;
+    });
+  }
+}