imports: fix a case where we weren't gofmt-compatible

Because goimports also sorts stdlib-vs-external differently
(whereas gofmt just sorts string-wise), we need to put a blank
line between imports of different classes. This happened in
some cases, but not all.

Fixes golang/go#7132

LGTM=kamil.kisiel
R=golang-codereviews, kamil.kisiel
CC=golang-codereviews
https://golang.org/cl/60870047

imports/fix_test.go

@@ -469,6 +469,42 @@ import "fmt"
 func main() {
 	fmt.Println("Hello, world")
 }
+`,
+	},
+
+	// golang.org/issue/7132
+	{
+		name: "issue 7132",
+		in: `package main
+
+import (
+"fmt"
+
+"gu"
+"github.com/foo/bar"
+)
+
+var (
+a = bar.a
+b = gu.a
+c = fmt.Printf
+)
+`,
+		out: `package main
+
+import (
+	"fmt"
+
+	"gu"
+
+	"github.com/foo/bar"
+)
+
+var (
+	a = bar.a
+	b = gu.a
+	c = fmt.Printf
+)
 `,
 	},
 }
@@ -492,7 +528,7 @@ func TestFixImports(t *testing.T) {
 		if *only != "" && tt.name != *only {
 			continue
 		}
-		buf, err := Process("foo.go", []byte(tt.in), nil)
+		buf, err := Process(tt.name+".go", []byte(tt.in), nil)
 		if err != nil {
 			t.Errorf("error on %q: %v", tt.name, err)
 			continue

imports/imports.go

@@ -54,10 +54,13 @@ func Process(filename string, src []byte, opt *Options) ([]byte, error) {
 	imps := astutil.Imports(fileSet, file)
 
 	var spacesBefore []string // import paths we need spaces before
-	if len(imps) == 1 {
+	for _, impSection := range imps {
-		// We have just one block of imports. See if any are in different groups numbers.
+		// Within each block of contiguous imports, see if any
+		// import lines are in different group numbers. If so,
+		// we'll need to put a space between them so it's
+		// compatible with gofmt.
 		lastGroup := -1
-		for _, importSpec := range imps[0] {
+		for _, importSpec := range impSection {
 			importPath, _ := strconv.Unquote(importSpec.Path.Value)
 			groupNum := importGroup(importPath)
 			if groupNum != lastGroup && lastGroup != -1 {