Revert "bufio: make Reader.Peek invalidate Unreads"

Go 1.9 beta 2 is already out.
It's too late to break code with a change like this.
This can be rolled forward for Go 1.10.

This reverts commit ae238688.

Change-Id: Ib67b8629e3deac5d50d76581aba6a91ca7a7853e
Reviewed-on: https://go-review.googlesource.com/47570Reviewed-by: Russ Cox <rsc@golang.org>

src/bufio/bufio.go

@@ -125,9 +125,6 @@ func (b *Reader) Peek(n int) ([]byte, error) {
 		return nil, ErrNegativeCount
 	}
 
-	b.lastByte = -1
-	b.lastRuneSize = -1
-
 	for b.w-b.r < n && b.w-b.r < len(b.buf) && b.err == nil {
 		b.fill() // b.w-b.r < len(b.buf) => buffer is not full
 	}

src/bufio/bufio_test.go

@@ -285,24 +285,6 @@ func TestUnreadRune(t *testing.T) {
 	}
 }
 
-func TestNoUnreadRuneAfterPeek(t *testing.T) {
-	br := NewReader(strings.NewReader("example"))
-	br.ReadRune()
-	br.Peek(1)
-	if err := br.UnreadRune(); err == nil {
-		t.Error("UnreadRune didn't fail after Peek")
-	}
-}
-
-func TestNoUnreadByteAfterPeek(t *testing.T) {
-	br := NewReader(strings.NewReader("example"))
-	br.ReadByte()
-	br.Peek(1)
-	if err := br.UnreadByte(); err == nil {
-		t.Error("UnreadByte didn't fail after Peek")
-	}
-}
-
 func TestUnreadByte(t *testing.T) {
 	segments := []string{"Hello, ", "world"}
 	r := NewReader(&StringReader{data: segments})