json: allow using '$' and '-' as the struct field's tag

R=adg, rsc, bradfitz, mattn.jp, gustavo
CC=golang-dev
https://golang.org/cl/4625081

src/pkg/json/decode_test.go

@@ -40,13 +40,6 @@ var (
 	umtrue   = unmarshaler{true}
 )
 
-type badTag struct {
-	X string
-	Y string `json:"y"`
-	Z string `x:"@#*%(#@"`
-	W string `json:"@#$@#$"`
-}
-
 type unmarshalTest struct {
 	in  string
 	ptr interface{}
@@ -68,9 +61,6 @@ var unmarshalTests = []unmarshalTest{
 	{`{"X": [1,2,3], "Y": 4}`, new(T), T{Y: 4}, &UnmarshalTypeError{"array", reflect.TypeOf("")}},
 	{`{"x": 1}`, new(tx), tx{}, &UnmarshalFieldError{"x", txType, txType.Field(0)}},
 
-	// skip invalid tags
-	{`{"X":"a", "y":"b", "Z":"c", "W":"d"}`, new(badTag), badTag{"a", "b", "c", "d"}, nil},
-
 	// syntax errors
 	{`{"X": "foo", "Y"}`, nil, nil, &SyntaxError{"invalid character '}' after object key", 17}},