No optimization for js

Author: yglukhov

lib/system.nim

@@ -1472,7 +1472,7 @@ when defined(nimHasTopDownInference):
   proc `@`*[T](a: openArray[T]): seq[T] {.magic: "OpenArrayToSeq".} =
     ## Turns an *openArray* into a sequence.
     let sz = a.len
-    when supportsCopyMem(T):
+    when supportsCopyMem(T) and not defined(js):
       result = newSeqUninit[T](sz)
       when nimvm:
         for i in 0..sz-1: result[i] = a[i]