encoding/json: disable anonymous fields

We should, after Go 1, make them work the same as
package xml, that is, make them appear in the outer
struct.  For now turn them off so that people do not
depend on the old behavior.

Fixing them is issue 3069.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5656102

src/pkg/encoding/json/encode.go

@@ -538,6 +538,11 @@ func encodeFields(t reflect.Type) []encodeField {
 		if f.PkgPath != "" {
 			continue
 		}
+		if f.Anonymous {
+			// We want to do a better job with these later,
+			// so for now pretend they don't exist.
+			continue
+		}
 		var ef encodeField
 		ef.i = i
 		ef.tag = f.Name