cmd/build: reject non-existant directories in ImportDir

Re-apply @adg's CL https://golang.org/cl/7129048 that was
previously disabled in https://golang.org/cl/7235052 because
it broke `godoc net/http` for go1.1.

Currently `godoc net/http` seems to work fine with this CL.

Fixes #3428.

Change-Id: I7df06df02fd62dededac6ec60bea62561be59cf1
Reviewed-on: https://go-review.googlesource.com/23013
Run-TryBot: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>

src/go/build/build.go

@@ -403,7 +403,11 @@ 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/go/build/build_test.go

@@ -5,6 +5,7 @@
 package build
 
 import (
+	"fmt"
 	"internal/testenv"
 	"io"
 	"os"
@@ -345,3 +346,13 @@ func TestImportVendorParentFailure(t *testing.T) {
 		t.Fatalf("error on failed import does not mention GOROOT/src/vendor directory:\n%s", e)
 	}
 }
+
+// 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", filepath.FromSlash(dir))
+	if err == nil || err.Error() != want {
+		t.Errorf("got error %q, want %q", err, want)
+	}
+}