go/build: ImportDir reject directories that don't exist

Fixes #3428.

R=dave, remyoudompheng, rsc
CC=golang-dev
https://golang.org/cl/7129048

src/pkg/go/build/build.go

@@ -321,7 +321,11 @@ func (p *Package) IsCommand() bool {
 // ImportDir is like Import but processes the Go package found in
 // the named directory.
 func (ctxt *Context) ImportDir(dir string, mode ImportMode) (*Package, error) {
-	return ctxt.Import(".", dir, mode)
+	p, err := ctxt.Import(".", dir, mode)
+	if err == nil && !ctxt.isDir(p.Dir) {
+		err = fmt.Errorf("%q is not a directory", p.Dir)
+	}
+	return p, err
 }
 
 // NoGoError is the error used by Import to describe a directory

src/pkg/go/build/build_test.go

@@ -5,6 +5,7 @@
 package build
 
 import (
+	"fmt"
 	"os"
 	"path/filepath"
 	"runtime"
@@ -89,6 +90,16 @@ func TestLocalDirectory(t *testing.T) {
 	}
 }
 
+// golang.org/issue/3248
+func TestBogusDirectory(t *testing.T) {
+	const dir = "/foo/bar/baz/gopher"
+	_, err := ImportDir(dir, FindOnly)
+	want := fmt.Sprintf("%q is not a directory", dir)
+	if err == nil || err.Error() != want {
+		t.Error("got error %q, want %q", err, want)
+	}
+}
+
 func TestShouldBuild(t *testing.T) {
 	const file1 = "// +build tag1\n\n" +
 		"package main\n"