go/parser: fix example to run on the playground

The example shouldn't rely on the existance of example_test.go. That
breaks in the playground, which is what the "run" button in
https://golang.org/pkg/go/parser/#example_ParseFile does.

Make the example self-sufficient by using a small piece of source via a
string literal instead.

Fixes #19823.

Change-Id: Ie8a3c6c5d00724e38ff727862b62e6a3621adc88
Reviewed-on: https://go-review.googlesource.com/39236
Run-TryBot: Daniel Martí <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>

Author: mvdan

src/go/parser/example_test.go

@@ -13,9 +13,19 @@ import (
 func ExampleParseFile() {
 	fset := token.NewFileSet() // positions are relative to fset
 
-	// Parse the file containing this very example
-	// but stop after processing the imports.
-	f, err := parser.ParseFile(fset, "example_test.go", nil, parser.ImportsOnly)
+	src := `package foo
+
+import (
+	"fmt"
+	"time"
+)
+
+func bar() {
+	fmt.Println(time.Now())
+}`
+
+	// Parse src but stop after processing the imports.
+	f, err := parser.ParseFile(fset, "", src, parser.ImportsOnly)
 	if err != nil {
 		fmt.Println(err)
 		return
@@ -29,6 +39,5 @@ func ExampleParseFile() {
 	// output:
 	//
 	// "fmt"
-	// "go/parser"
-	// "go/token"
+	// "time"
 }