Convert assert.go back to assert.star and bump Go version to 1.16. (

#453)

* Convert `assert.go` back to `assert.star`.

* Add newline to `assert.star`.

* Add back `DataFile` public API.

go.mod

@@ -1,6 +1,6 @@
 module go.starlark.net
 
-go 1.13
+go 1.16
 
 require (
 	github.com/chzyer/logex v1.1.10 // indirect

starlarktest/assert.go → starlarktest/assert.star

@@ -1,9 +1,3 @@
-package starlarktest
-
-// assertStar contains the logical data file assert.star.
-// TODO(adonovan): move it back into an actual file,
-// when fs.Embed is more than two releases old.
-const assertStar = `
 # Predeclared built-ins for this module:
 #
 # error(msg): report an error in Go's test framework without halting execution.
@@ -55,4 +49,3 @@ assert = module(
     contains = _contains,
     fails = _fails,
 )
-`

starlarktest/starlarktest.go

@@ -13,6 +13,7 @@
 package starlarktest // import "go.starlark.net/starlarktest"
 
 import (
+	_ "embed"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -50,9 +51,11 @@ func GetReporter(thread *starlark.Thread) Reporter {
 }
 
 var (
-	once      sync.Once
-	assert    starlark.StringDict
-	assertErr error
+	once   sync.Once
+	assert starlark.StringDict
+	//go:embed assert.star
+	assertFileSrc string
+	assertErr     error
 )
 
 // LoadAssertModule loads the assert module.
@@ -66,10 +69,8 @@ func LoadAssertModule() (starlark.StringDict, error) {
 			"module":  starlark.NewBuiltin("module", starlarkstruct.MakeModule),
 			"_freeze": starlark.NewBuiltin("freeze", freeze),
 		}
-		// TODO(adonovan): embed the file using embed.FS when we can rely on go1.16,
-		// and make the apparent filename reference that file.
 		thread := new(starlark.Thread)
-		assert, assertErr = starlark.ExecFile(thread, "builtins/assert.star", assertStar, predeclared)
+		assert, assertErr = starlark.ExecFile(thread, "assert.star", assertFileSrc, predeclared)
 	})
 	return assert, assertErr
 }