cmd/go: handle paths like \x.go on windows

Fixes #8130.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews
https://golang.org/cl/143200043

src/cmd/go/build.go

@@ -505,8 +505,13 @@ func goFilesPackage(gofiles []string) *Package {
 	}
 	ctxt.ReadDir = func(string) ([]os.FileInfo, error) { return dirent, nil }
 
-	if !filepath.IsAbs(dir) {
-		dir = filepath.Join(cwd, dir)
+	var err error
+	if dir == "" {
+		dir = cwd
+	}
+	dir, err = filepath.Abs(dir)
+	if err != nil {
+		fatalf("%s", err)
 	}
 
 	bp, err := ctxt.ImportDir(dir, 0)

src/cmd/go/go_windows_test.go

+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestAbsolutePath(t *testing.T) {
+	tmp, err := ioutil.TempDir("", "TestAbsolutePath")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmp)
+
+	file := filepath.Join(tmp, "a.go")
+	err = ioutil.WriteFile(file, []byte{}, 0644)
+	if err != nil {
+		t.Fatal(err)
+	}
+	dir := filepath.Join(tmp, "dir")
+	err = os.Mkdir(dir, 0777)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	wd, err := os.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.Chdir(wd)
+
+	// Chdir so current directory and a.go reside on the same drive.
+	err = os.Chdir(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	noVolume := file[len(filepath.VolumeName(file)):]
+	wrongPath := filepath.Join(dir, noVolume)
+	output, err := exec.Command("go", "build", noVolume).CombinedOutput()
+	if err == nil {
+		t.Fatal("build should fail")
+	}
+	if strings.Contains(string(output), wrongPath) {
+		t.Fatalf("wrong output found: %v %v", err, string(output))
+	}
+}