encoding/json: roll back Unmarshal optimization + test

The second attempt at the Unmarshal optimization allowed
panics to get out of the json package. Add test for that bug
and remove the optimization.

Let's stop trying to optimize Unmarshal.

Fixes #4784.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/7300108

src/pkg/encoding/json/decode_test.go

@@ -11,6 +11,7 @@ import (
 	"reflect"
 	"strings"
 	"testing"
+	"time"
 )
 
 type T struct {
@@ -1113,3 +1114,30 @@ func TestUnmarshalUnexported(t *testing.T) {
 		t.Errorf("got %q, want %q", out, want)
 	}
 }
+
+// Time3339 is a time.Time which encodes to and from JSON
+// as an RFC 3339 time in UTC.
+type Time3339 time.Time
+
+func (t *Time3339) UnmarshalJSON(b []byte) error {
+	if len(b) < 2 || b[0] != '"' || b[len(b)-1] != '"' {
+		return fmt.Errorf("types: failed to unmarshal non-string value %q as an RFC 3339 time")
+	}
+	tm, err := time.Parse(time.RFC3339, string(b[1:len(b)-1]))
+	if err != nil {
+		return err
+	}
+	*t = Time3339(tm)
+	return nil
+}
+
+func TestUnmarshalJSONLiteralError(t *testing.T) {
+	var t3 Time3339
+	err := Unmarshal([]byte(`"0000-00-00T00:00:00Z"`), &t3)
+	if err == nil {
+		t.Fatalf("expected error; got time %v", time.Time(t3))
+	}
+	if !strings.Contains(err.Error(), "range") {
+		t.Errorf("got err = %v; want out of range error", err)
+	}
+}