completed faststart support with async methods

source.js

@@ -108,40 +108,43 @@ class Source extends Resource {
     this.byteLength = opts.byteLength || 0
   }
 
-  isFaststart() {
-    if (
-      [path.extname(this.uri), path.extname(this.pathname)].includes('.m4a') ||
-      [path.extname(this.uri), path.extname(this.pathname)].includes('.mp4') ||
-      [path.extname(this.uri), path.extname(this.pathname)].includes('.mov')
-    ) {
-      if (!this.buffer) {
-        return new Error('No buffer in `Source`')
-      }
-      return isFaststart(this.buffer)
-    } else {
-      return
-    }
+  /**
+   * Answers the age-old question: is my MPEG-4 container faststarted?
+   * @async
+   * @returns {Boolean}
+   */
+  async isFaststart() {
+    return isFaststart(await this.toBuffer())
   }
 
-  faststart() {
-    if (!this.buffer) {
-      return new Error('No buffer in `Source`')
-    }
-
+  /**
+   * Create a faststart buffer from the Source
+   * @async
+   * @returns {Buffer}
+   */
+  async faststart() {
     try {
-      return faststart(this.buffer)
+      if (
+        [path.extname(this.uri), path.extname(this.pathname)].includes('.m4a') ||
+        [path.extname(this.uri), path.extname(this.pathname)].includes('.mp4') ||
+        [path.extname(this.uri), path.extname(this.pathname)].includes('.mov')
+      ) {
+      return faststart(await this.toBuffer())
+      } else {
+      return
+    }
     } catch (err) {
       return err
     }
   }
 
-  toBuffer(cb) {
-    streamToBuffer(this.createReadStream())
-      .then(buf => {
-        this.buffer = buf
-        cb(null, buf)
-      })
-      .catch(err => cb(err))
+  /**
+   * Create a buffer from the Source
+   * @async
+   * @returns {Buffer}
+   */
+  async toBuffer() {
+    return await streamToBuffer(this.createReadStream())
   }
 
   /**