go/build: reject empty strings in Import

Fixes #3889.

R=rsc, adg
CC=golang-dev
https://golang.org/cl/6499102

src/pkg/go/build/build.go

@@ -351,6 +351,9 @@ func (ctxt *Context) Import(path string, srcDir string, mode ImportMode) (*Packa
 	p := &Package{
 		ImportPath: path,
 	}
+	if path == "" {
+		return p, fmt.Errorf("import %q: invalid import path", path)
+	}
 
 	var pkga string
 	var pkgerr error

src/pkg/go/build/build_test.go

@@ -61,6 +61,19 @@ func TestDotSlashImport(t *testing.T) {
 	}
 }
 
+func TestEmptyImport(t *testing.T) {
+	p, err := Import("", Default.GOROOT, FindOnly)
+	if err == nil {
+		t.Fatal(`Import("") returned nil error.`)
+	}
+	if p == nil {
+		t.Fatal(`Import("") returned nil package.`)
+	}
+	if p.ImportPath != "" {
+		t.Fatalf("ImportPath=%q, want %q.", p.ImportPath, "")
+	}
+}
+
 func TestLocalDirectory(t *testing.T) {
 	cwd, err := os.Getwd()
 	if err != nil {