buffer: initialize TextDecoder once on blob for performance

Author: anonrig

lib/internal/blob.js

@@ -78,6 +78,7 @@ let ReadableStream;
 let URL;
 
 const enc = new TextEncoder();
+const dec = new TextDecoder();
 
 // Yes, lazy loading is annoying but because of circular
 // references between the url, internal/blob, and buffer
@@ -310,8 +311,6 @@ class Blob {
   async text() {
     if (!isBlob(this))
       throw new ERR_INVALID_THIS('Blob');
-
-    const dec = new TextDecoder();
     return dec.decode(await this.arrayBuffer());
   }