go/ast/astutil: do not remove imports in AddNamedImport

When adding imports to an existing import declaration, ensure that
the target declaration will be printed with parentheses. This allows
all of the imported packages to be printed, not just the first one.

Fixes golang/go#17212

Change-Id: Ie5de5ec9bca6169650336ee2ea98334817e64e48
Reviewed-on: https://go-review.googlesource.com/29688
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

go/ast/astutil/imports.go

@@ -160,6 +160,9 @@ func AddNamedImport(fset *token.FileSet, f *ast.File, name, ipath string) (added
 			first = gen
 			continue // Don't touch the first one.
 		}
+		// We now know there is more than one package in this import
+		// declaration. Ensure that it ends up parenthesized.
+		first.Lparen = first.Pos()
 		// Move the imports of the other import declaration to the first one.
 		for _, spec := range gen.Specs {
 			spec.(*ast.ImportSpec).Path.ValuePos = first.Pos()

go/ast/astutil/imports_test.go

@@ -509,6 +509,42 @@ import "C"
 // Comment
 import "C"
 import "bufio"
+`,
+	},
+	{
+		name: `issue 17212 several single-import lines with shared prefix ending in a slash`,
+		pkg:  "net/http",
+		in: `package main
+
+import "bufio"
+import "net/url"
+`,
+		out: `package main
+
+import (
+	"bufio"
+	"net/http"
+	"net/url"
+)
+`,
+	},
+	{
+		name: `issue 17212 block imports lines with shared prefix ending in a slash`,
+		pkg:  "net/http",
+		in: `package main
+
+import (
+	"bufio"
+	"net/url"
+)
+`,
+		out: `package main
+
+import (
+	"bufio"
+	"net/http"
+	"net/url"
+)
 `,
 	},
 	{