Sever isolate package's dependency on client/internal.

client/isolate/utils.go

 // Copyright 2015 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 package isolate
 
 import (
         "fmt"
         "log"
         "path"
+        "runtime"
         "strings"
 
         "github.com/luci/luci-go/common/errors"
 )
 
 // TODO(tandrii): Remove this hacky stuff.
 // hacky stuff for faster debugging.
 func assertNoError(err error) {
         if err == nil {
                 return
@@ skipping 105 matching lines @@
                         n += 3
                 }
                 if t0 != tl {
                         buf[n] = '/'
                         copy(buf[n+1:], targ[t0:])
                 }
                 return string(buf), nil
         }
         return targ[t0:], nil
 }
+
+// IsWindows returns True when running on the best OS there is.
+func IsWindows() bool {
+        return runtime.GOOS == "windows"
+}