go/loader: use build.AllowVendor during Import on Go 1.6 and later

Makes programs like ssadump work on packages using vendored code,
for example net/http.

For golang/go#12278.
Depends on CL 17726 in main repository.

Change-Id: Ibabf564e397044a0f449087124dd96161081baaf
Reviewed-on: https://go-review.googlesource.com/17727
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

go/loader/loader.go

@@ -458,8 +458,9 @@ func (conf *Config) Load() (*Program, error) {
 		conf.FindPackage = func(ctxt *build.Context, path string) (*build.Package, error) {
 			// TODO(adonovan): cache calls to build.Import
 			// so we don't do it three times per test package.
+			// (Note that this is difficult due to vendoring.)
 			ioLimit <- true
-			bp, err := ctxt.Import(path, conf.Cwd, 0)
+			bp, err := ctxt.Import(path, conf.Cwd, importMode)
 			<-ioLimit
 			if _, ok := err.(*build.NoGoError); ok {
 				return bp, nil // empty directory is not an error

go/loader/loader_go15.go

@@ -0,0 +1,11 @@
+// Copyright 2015 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.
+
+// +build !go1.6
+
+package loader
+
+import "go/build"
+
+var importMode build.ImportMode = 0

go/loader/loader_go16.go

@@ -0,0 +1,11 @@
+// Copyright 2015 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.
+
+// +build go1.6
+
+package loader
+
+import "go/build"
+
+var importMode build.ImportMode = build.AllowVendor