exp/template/html: fix bug in cssEscaper

cssEscaper escapes using the CSS convention: `\` + hex + optional-space

It outputs the space when the escape could be followed by
a hex digit to distinguish a "\na" from "\u00aa".

It did not output a space when the escape is followed by a space
character so did not distinguish "\n " from "\n".

Currently when doing lookahead, it does not distinguish spaces that
will be escaped later by the same function from ones that will not.
This is correct but suboptimal.

R=nigeltao
CC=golang-dev
https://golang.org/cl/5306042

src/pkg/exp/template/html/css.go

@@ -144,6 +144,15 @@ func skipCSSSpace(c []byte) []byte {
 	return c
 }
 
+// isCSSSpace returns whether b is a CSS space char as defined in wc.
+func isCSSSpace(b byte) bool {
+	switch b {
+	case '\t', '\n', '\f', '\r', ' ':
+		return true
+	}
+	return false
+}
+
 // cssEscaper escapes HTML and CSS special characters using \<hex>+ escapes.
 func cssEscaper(args ...interface{}) string {
 	s, _ := stringify(args...)
@@ -198,7 +207,7 @@ func cssEscaper(args ...interface{}) string {
 		b.WriteString(s[written:i])
 		b.WriteString(repl)
 		written = i + utf8.RuneLen(r)
-		if repl != `\\` && (written == len(s) || isHex(s[written])) {
+		if repl != `\\` && (written == len(s) || isHex(s[written]) || isCSSSpace(s[written])) {
 			b.WriteByte(' ')
 		}
 	}

src/pkg/exp/template/html/css_test.go

@@ -100,9 +100,13 @@ func TestDecodeCSS(t *testing.T) {
 		},
 	}
 	for _, test := range tests {
-		got := string(decodeCSS([]byte(test.css)))
-		if got != test.want {
-			t.Errorf("%q: want\n\t%q\nbut got\n\t%q", test.css, test.want, got)
+		got1 := string(decodeCSS([]byte(test.css)))
+		if got1 != test.want {
+			t.Errorf("%q: want\n\t%q\nbut got\n\t%q", test.css, test.want, got1)
+		}
+		recoded := cssEscaper(got1)
+		if got2 := string(decodeCSS([]byte(recoded))); got2 != test.want {
+			t.Errorf("%q: escape & decode not dual for %q", test.css, recoded)
 		}
 	}
 }
@@ -156,7 +160,7 @@ func TestCSSEscaper(t *testing.T) {
 		"\u00A0\u0100\u2028\u2029\ufeff\U0001D11E")
 
 	want := ("\\0\x01\x02\x03\x04\x05\x06\x07" +
-		"\x08\\9\\a\x0b\\c\\d\x0E\x0F" +
+		"\x08\\9 \\a\x0b\\c \\d\x0E\x0F" +
 		"\x10\x11\x12\x13\x14\x15\x16\x17" +
 		"\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f" +
 		` !\22#$%\26\27\28\29*\2b,-.\2f ` +