diff --git a/go/buildutil/util.go b/go/buildutil/util.go
new file mode 100644
index 0000000000..f4dca6a234
--- /dev/null
+++ b/go/buildutil/util.go
@@ -0,0 +1,127 @@
+// 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 buildutil
+
+import (
+	"fmt"
+	"go/ast"
+	"go/build"
+	"go/parser"
+	"go/token"
+	"io"
+	"os"
+	"path"
+	"path/filepath"
+	"strings"
+)
+
+// ParseFile behaves like parser.ParseFile,
+// but uses the build context's virtual file system, if any.
+//
+// If file is not absolute (as defined by IsAbsPath), the (dir, file)
+// components are joined using JoinPath; dir must be absolute.
+//
+// The displayPath function, if provided, is used to transform the
+// filename that will be attached to the ASTs.
+//
+// TODO(adonovan): call this from go/loader.parseFiles when the tree thaws.
+//
+func ParseFile(fset *token.FileSet, ctxt *build.Context, displayPath func(string) string, dir string, file string, mode parser.Mode) (*ast.File, error) {
+	if !IsAbsPath(ctxt, file) {
+		file = JoinPath(ctxt, dir, file)
+	}
+	rd, err := OpenFile(ctxt, file)
+	if err != nil {
+		return nil, err
+	}
+	defer rd.Close() // ignore error
+	if displayPath != nil {
+		file = displayPath(file)
+	}
+	return parser.ParseFile(fset, file, rd, mode)
+}
+
+// ContainingPackage returns the package containing filename.
+//
+// If filename is not absolute, it is interpreted relative to working directory dir.
+// All I/O is via the build context's virtual file system, if any.
+//
+// The '...Files []string' fields of the resulting build.Package are not
+// populated (build.FindOnly mode).
+//
+// TODO(adonovan): call this from oracle when the tree thaws.
+//
+func ContainingPackage(ctxt *build.Context, dir, filename string) (*build.Package, error) {
+	if !IsAbsPath(ctxt, filename) {
+		filename = JoinPath(ctxt, dir, filename)
+	}
+
+	// We must not assume the virtualized file tree uses
+	// "/" always,
+	// `\` always,
+	// or os.PathSeparator (which varies by platform),
+	// but to make any progress, we are forced to assume that
+	// virtualized paths will not use `\` unless the PathSeparator
+	// is also `\`, thus we can rely on filepath.ToSlash for some sanity.
+
+	dirSlash := path.Dir(filepath.ToSlash(filename)) + "/"
+
+	// We assume that no source root (GOPATH[i] or GOROOT) contains any other.
+	for _, srcdir := range ctxt.SrcDirs() {
+		srcdirSlash := filepath.ToSlash(srcdir) + "/"
+		if strings.HasPrefix(dirSlash, srcdirSlash) {
+			importPath := dirSlash[len(srcdirSlash) : len(dirSlash)-len("/")]
+			return ctxt.Import(importPath, dir, build.FindOnly)
+		}
+	}
+
+	return nil, fmt.Errorf("can't find package containing %s", filename)
+}
+
+// -- Effective methods of virtual file system -------------------------
+
+// FileExists returns true if the specified file exists,
+// using the build context's virtual file system.
+func FileExists(ctxt *build.Context, path string) bool {
+	if ctxt.OpenFile != nil {
+		r, err := ctxt.OpenFile(path)
+		if err != nil {
+			return false
+		}
+		r.Close() // ignore error
+		return true
+	}
+	_, err := os.Stat(path)
+	return err == nil
+}
+
+// OpenFile behaves like os.Open,
+// but uses the build context's virtual file system, if any.
+func OpenFile(ctxt *build.Context, path string) (io.ReadCloser, error) {
+	if ctxt.OpenFile != nil {
+		return ctxt.OpenFile(path)
+	}
+	return os.Open(path)
+}
+
+// IsAbsPath behaves like filepath.IsAbs,
+// but uses the build context's virtual file system, if any.
+func IsAbsPath(ctxt *build.Context, path string) bool {
+	if ctxt.IsAbsPath != nil {
+		return ctxt.IsAbsPath(path)
+	}
+	return filepath.IsAbs(path)
+}
+
+// JoinPath behaves like filepath.Join,
+// but uses the build context's virtual file system.
+func JoinPath(ctxt *build.Context, path ...string) string {
+	if ctxt.JoinPath != nil {
+		return ctxt.JoinPath(path...)
+	}
+	return filepath.Join(path...)
+}
+
+// TODO(adonovan): SplitPathList, IsDir, HasSubdir, ReadDir?
diff --git a/go/buildutil/util_test.go b/go/buildutil/util_test.go
new file mode 100644
index 0000000000..f0db26c794
--- /dev/null
+++ b/go/buildutil/util_test.go
@@ -0,0 +1,41 @@
+// 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 buildutil_test
+
+import (
+	"go/build"
+	"os"
+	"path/filepath"
+	"runtime"
+	"testing"
+
+	"code.google.com/p/go.tools/go/buildutil"
+)
+
+func TestContainingPackage(t *testing.T) {
+	// unvirtualized:
+	goroot := runtime.GOROOT()
+	gopath := filepath.SplitList(os.Getenv("GOPATH"))[0]
+
+	for _, test := range [][2]string{
+		{goroot + "/src/fmt/print.go", "fmt"},
+		{goroot + "/src/encoding/json/foo.go", "encoding/json"},
+		{goroot + "/src/encoding/missing/foo.go", "(not found)"},
+		{gopath + "/src/code.google.com/p/go.tools/go/buildutil/util_test.go",
+			"code.google.com/p/go.tools/go/buildutil"},
+	} {
+		file, want := test[0], test[1]
+		bp, err := buildutil.ContainingPackage(&build.Default, ".", file)
+		got := bp.ImportPath
+		if err != nil {
+			got = "(not found)"
+		}
+		if got != want {
+			t.Errorf("ContainingPackage(%q) = %s, want %s", file, got, want)
+		}
+	}
+
+	// TODO(adonovan): test on virtualized GOPATH too.
+}