unicode/utf8: remove dependence on unicode.

The dependency was there only to pull in two constants.
Now we define them locally and verify equality in the test.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5754046

src/pkg/unicode/utf8/utf8.go

@@ -6,12 +6,15 @@
 // UTF-8. It includes functions to translate between runes and UTF-8 byte sequences.
 package utf8
 
-import "unicode" // only needed for a couple of constants
+// The conditions RuneError==unicode.ReplacementChar and
+// MaxRune==unicode.MaxRune are verified in the tests.
+// Defining them locally avoids this package depending on package unicode.
 
 // Numbers fundamental to the encoding.
 const (
-	RuneError = unicode.ReplacementChar // the "error" Rune or "replacement character".
+	RuneError = '\uFFFD'     // the "error" Rune or "Unicode replacement character"
 	RuneSelf  = 0x80         // characters below Runeself are represented as themselves in a single byte.
+	MaxRune   = '\U0010FFFF' // Maximum valid Unicode code point.
 	UTFMax    = 4            // maximum number of bytes of a UTF-8 encoded Unicode character.
 )
 
@@ -309,7 +312,7 @@ func EncodeRune(p []byte, r rune) int {
 		return 2
 	}
 
-	if uint32(r) > unicode.MaxRune {
+	if uint32(r) > MaxRune {
 		r = RuneError
 	}
 

src/pkg/unicode/utf8/utf8_test.go

@@ -7,9 +7,20 @@ package utf8_test
 import (
 	"bytes"
 	"testing"
+	"unicode"
 	. "unicode/utf8"
 )
 
+// Validate the constants redefined from unicode.
+func init() {
+	if MaxRune != unicode.MaxRune {
+		panic("utf8.MaxRune is wrong")
+	}
+	if RuneError != unicode.ReplacementChar {
+		panic("utf8.RuneError is wrong")
+	}
+}
+
 type Utf8Map struct {
 	r   rune
 	str string