go/ast/astutil: set Pos for import name

Failing to set this when adding a named import
to an existing block at the 0th position
caused the Lparen position to be set to zero.
As a result, the specs were printed as if
they were a single spec, not a group.
This made it appear as if imports had
been swallowed.

See CL 8663 for more context
and the original bug report.

CL 2050 fixed most similar cases
but missed this one.

Change-Id: Ic578fbb8040fa3d3d41db5bde2b839e394801608
Reviewed-on: https://go-review.googlesource.com/10252
Reviewed-by: Robert Griesemer <gri@golang.org>

go/ast/astutil/imports.go

@@ -121,6 +121,9 @@ func AddNamedImport(fset *token.FileSet, f *ast.File, name, ipath string) (added
 		// so that the sorter sees it as being in the same block.
 		pos = impDecl.Specs[insertAt-1].Pos()
 	}
+	if newImport.Name != nil {
+		newImport.Name.NamePos = pos
+	}
 	newImport.Path.ValuePos = pos
 	newImport.EndPos = pos
 

go/ast/astutil/imports_test.go

@@ -378,6 +378,22 @@ import (
 	}
 }
 
+func TestDoubleAddNamedImport(t *testing.T) {
+	file := parse(t, "doublenamedimport", "package main\n")
+	AddNamedImport(fset, file, "o", "os")
+	AddNamedImport(fset, file, "i", "io")
+	want := `package main
+
+import (
+	i "io"
+	o "os"
+)
+`
+	if got := print(t, "doublenamedimport", file); got != want {
+		t.Errorf("got: %s\nwant: %s", got, want)
+	}
+}
+
 // Part of issue 8729.
 func TestDoubleAddImportWithDeclComment(t *testing.T) {
 	file := parse(t, "doubleimport", `package main