go/doc: fix build

1) go/doc:
   - create correct ast.FuncType
   - use more commonly used variable names in a test case

2) make ast.FuncType.Pos robust in case of incorrect ASTs

R=golang-dev
CC=golang-dev
https://golang.org/cl/9651044

src/pkg/go/ast/ast.go

@@ -439,7 +439,7 @@ func (x *KeyValueExpr) Pos() token.Pos   { return x.Key.Pos() }
 func (x *ArrayType) Pos() token.Pos      { return x.Lbrack }
 func (x *StructType) Pos() token.Pos     { return x.Struct }
 func (x *FuncType) Pos() token.Pos {
-	if x.Func.IsValid() {
+	if x.Func.IsValid() || x.Params == nil { // see issue 3870
 		return x.Func
 	}
 	return x.Params.Pos() // interface method declarations have no "func" keyword

src/pkg/go/doc/example.go

@@ -265,7 +265,7 @@ func playExample(file *ast.File, body *ast.BlockStmt) *ast.File {
 	// Synthesize main function.
 	funcDecl := &ast.FuncDecl{
 		Name: ast.NewIdent("main"),
-		Type: &ast.FuncType{},
+		Type: &ast.FuncType{Params: &ast.FieldList{}}, // FuncType.Params must be non-nil
 		Body: body,
 	}
 

src/pkg/go/doc/example_test.go

@@ -159,8 +159,8 @@ func main() {
 `
 
 func TestExamples(t *testing.T) {
-	fs := token.NewFileSet()
-	file, err := parser.ParseFile(fs, "test.go", strings.NewReader(exampleTestFile), parser.ParseComments)
+	fset := token.NewFileSet()
+	file, err := parser.ParseFile(fset, "test.go", strings.NewReader(exampleTestFile), parser.ParseComments)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -174,11 +174,11 @@ func TestExamples(t *testing.T) {
 			if e.Play == nil {
 				g = "<nil>"
 			} else {
-				b := new(bytes.Buffer)
-				if err := format.Node(b, fs, e.Play); err != nil {
+				var buf bytes.Buffer
+				if err := format.Node(&buf, fset, e.Play); err != nil {
 					t.Fatal(err)
 				}
-				g = b.String()
+				g = buf.String()
 			}
 			if g != w {
 				t.Errorf("%s: got Play == %q, want %q", c.Name, g, w)