go.tools/imports: go/format outbound code

Fixes golang/go#8035.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews, shurcool
https://golang.org/cl/120840044

imports/fix_test.go

@@ -616,6 +616,33 @@ import (
 func main() {
 	_, _, _, _, _ = fmt.Errorf, io.Copy, strings.Contains, renamed_bar.A, B
 }
+`,
+	},
+
+	// Non-idempotent comment formatting
+	// golang.org/issue/8035
+	{
+		name: "issue 8035",
+		in: `package main
+
+import (
+	"fmt"                     // A
+	"go/ast"                  // B
+	_ "launchpad.net/gocheck" // C
+)
+
+func main() { _, _ = fmt.Print, ast.Walk }
+`,
+		out: `package main
+
+import (
+	"fmt"    // A
+	"go/ast" // B
+
+	_ "launchpad.net/gocheck" // C
+)
+
+func main() { _, _ = fmt.Print, ast.Walk }
 `,
 	},
 }

imports/imports.go

@@ -11,6 +11,7 @@ import (
 	"bytes"
 	"fmt"
 	"go/ast"
+	"go/format"
 	"go/parser"
 	"go/printer"
 	"go/token"
@@ -89,6 +90,11 @@ func Process(filename string, src []byte, opt *Options) ([]byte, error) {
 	if len(spacesBefore) > 0 {
 		out = addImportSpaces(bytes.NewReader(out), spacesBefore)
 	}
+
+	out, err = format.Source(out)
+	if err != nil {
+		return nil, err
+	}
 	return out, nil
 }