playground: Add missing supported packages.

In commit ef99944, I've run update.sh to generate the latest playground. In the diff, it's visible that 3 files were removed:

-	playground/pkg/encoding/gob.a.js
-	playground/pkg/go/build.a.js
-	playground/pkg/go/doc.a.js

That means they were previously built but not anymore with the latest version (possibly because the imports graph of standard library has changed in Go 1.5, so they are no longer dependencies, or possibly they were included erroneously). They are not explicitly listed in the list of packages to install and include.

According to the current compatibility table, 2 of those 3 packages are supported (encoding/gob and go/doc). Add them to the update.sh script so they are included.

Also add go/ast explicitly since it's supported. It was already built (as a dependency), but this will ensure it'll always be included even if it's no longer a dependency.

Author: dmitshur

Diff for: playground/pkg/encoding/gob.a.js

@@ -15,7 +15,7 @@ mkdir -p $PKG/github.com/gopherjs/gopherjs
 cp $GOPATH/pkg/darwin_js_min/github.com/gopherjs/gopherjs/js.a $PKG/github.com/gopherjs/gopherjs/js.a
 cp $GOPATH/pkg/darwin_js_min/github.com/gopherjs/gopherjs/nosync.a $PKG/github.com/gopherjs/gopherjs/nosync.a
 
-gopherjs install -m github.com/gopherjs/gopherjs/js archive/tar archive/zip bufio bytes compress/bzip2 compress/flate compress/gzip compress/lzw compress/zlib container/heap container/list container/ring crypto/aes crypto/cipher crypto/des crypto/dsa crypto/ecdsa crypto/elliptic crypto/hmac crypto/md5 crypto/rand crypto/rc4 crypto/rsa crypto/sha1 crypto/sha256 crypto/sha512 crypto/subtle database/sql/driver debug/gosym debug/pe encoding/ascii85 encoding/asn1 encoding/base32 encoding/base64 encoding/binary encoding/csv encoding/hex encoding/json encoding/pem encoding/xml errors fmt go/format go/printer go/token hash/adler32 hash/crc32 hash/crc64 hash/fnv html html/template image image/color image/draw image/gif image/jpeg image/png index/suffixarray io io/ioutil math math/big math/cmplx math/rand mime net/http/cookiejar net/http/fcgi net/http/httptest net/http/httputil net/mail net/smtp net/textproto net/url path path/filepath reflect regexp regexp/syntax sort strconv strings sync/atomic testing testing/iotest testing/quick text/scanner text/tabwriter text/template text/template/parse time unicode unicode/utf16 unicode/utf8
+gopherjs install -m github.com/gopherjs/gopherjs/js archive/tar archive/zip bufio bytes compress/bzip2 compress/flate compress/gzip compress/lzw compress/zlib container/heap container/list container/ring crypto/aes crypto/cipher crypto/des crypto/dsa crypto/ecdsa crypto/elliptic crypto/hmac crypto/md5 crypto/rand crypto/rc4 crypto/rsa crypto/sha1 crypto/sha256 crypto/sha512 crypto/subtle database/sql/driver debug/gosym debug/pe encoding/ascii85 encoding/asn1 encoding/base32 encoding/base64 encoding/binary encoding/csv encoding/gob encoding/hex encoding/json encoding/pem encoding/xml errors fmt go/ast go/doc go/format go/printer go/token hash/adler32 hash/crc32 hash/crc64 hash/fnv html html/template image image/color image/draw image/gif image/jpeg image/png index/suffixarray io io/ioutil math math/big math/cmplx math/rand mime net/http/cookiejar net/http/fcgi net/http/httptest net/http/httputil net/mail net/smtp net/textproto net/url path path/filepath reflect regexp regexp/syntax sort strconv strings sync/atomic testing testing/iotest testing/quick text/scanner text/tabwriter text/template text/template/parse time unicode unicode/utf16 unicode/utf8
 cp -r $GOROOT/pkg/darwin_js_min/* $PKG
 cp -r $GOROOT/pkg/darwin_amd64_js_min/* $PKG