url: use SafeSet to filter known special protocols

Avoids a maintenance hazard when reviewers assume that
`hostlessProtocol` and `slashedProtocol` are disjoint.

The following may be counter-intuitive:

```js
// These objects seem to have no keys in common
const hostlessProtocol = { 'javascript': true };
const slashedProtocol = { 'http': true };
// A reasonable reviewer may assumes bothTrue is never truthy
function bothTrue(lowerProto) {
  return hostlessProtocol[lowerProto] && slashedProtocol[lowerProto];
}
// But
console.log(Boolean(bothTrue('constructor')));  // true
```

This change uses SafeSet instead of plain-old objects.

----

Rejected alternative:

We could have used object with a `null` prototype as lookup tables
so that `lowerProto` is never treated as a key into `Object.prototype`.

```js
const hostlessProtocol = { __proto__: null, 'javascript': true };
const slashedProtocol = { __proto__: null, 'http': true };

function bothTrue(lowerProto) {
  return hostlessProtocol[lowerProto] && slashedProtocol[lowerProto];
}

console.log(Boolean(bothTrue('constructor')));  // false
```

lib/url.js

@@ -26,6 +26,8 @@ const { toASCII } = process.binding('config').hasIntl ?
 
 const { hexTable } = require('internal/querystring');
 
+const { SafeSet } = require('internal/safe_globals');
+
 const {
   ERR_INVALID_ARG_TYPE
 } = require('internal/errors').codes;
@@ -76,28 +78,28 @@ const simplePathPattern = /^(\/\/?(?!\/)[^?\s]*)(\?[^\s]*)?$/;
 
 const hostnameMaxLen = 255;
 // protocols that can allow "unsafe" and "unwise" chars.
-const unsafeProtocol = {
-  'javascript': true,
-  'javascript:': true
-};
+const unsafeProtocol = new SafeSet([
+  'javascript',
+  'javascript:'
+]);
 // protocols that never have a hostname.
-const hostlessProtocol = {
-  'javascript': true,
-  'javascript:': true
-};
+const hostlessProtocol = new SafeSet([
+  'javascript',
+  'javascript:'
+]);
 // protocols that always contain a // bit.
-const slashedProtocol = {
-  'http': true,
-  'http:': true,
-  'https': true,
-  'https:': true,
-  'ftp': true,
-  'ftp:': true,
-  'gopher': true,
-  'gopher:': true,
-  'file': true,
-  'file:': true
-};
+const slashedProtocol = new SafeSet([
+  'http',
+  'http:',
+  'https',
+  'https:',
+  'ftp',
+  'ftp:',
+  'gopher',
+  'gopher:',
+  'file',
+  'file:'
+]);
 const {
   CHAR_SPACE,
   CHAR_TAB,
@@ -267,14 +269,14 @@ Url.prototype.parse = function parse(url, parseQueryString, slashesDenoteHost) {
   if (slashesDenoteHost || proto || hostPattern.test(rest)) {
     var slashes = rest.charCodeAt(0) === CHAR_FORWARD_SLASH &&
                   rest.charCodeAt(1) === CHAR_FORWARD_SLASH;
-    if (slashes && !(proto && hostlessProtocol[lowerProto])) {
+    if (slashes && !(proto && hostlessProtocol.has(lowerProto))) {
       rest = rest.slice(2);
       this.slashes = true;
     }
   }
 
-  if (!hostlessProtocol[lowerProto] &&
-      (slashes || (proto && !slashedProtocol[proto]))) {
+  if (!hostlessProtocol.has(lowerProto) &&
+      (slashes || (proto && !slashedProtocol.has(proto)))) {
 
     // there's a hostname.
     // the first instance of /, ?, ;, or # ends the host.
@@ -400,7 +402,7 @@ Url.prototype.parse = function parse(url, parseQueryString, slashesDenoteHost) {
 
   // now rest is set to the post-host stuff.
   // chop off any delim chars.
-  if (!unsafeProtocol[lowerProto]) {
+  if (!unsafeProtocol.has(lowerProto)) {
     // First, make 100% sure that any "autoEscape" chars get
     // escaped, even if encodeURIComponent doesn't think they
     // need to be.
@@ -447,7 +449,7 @@ Url.prototype.parse = function parse(url, parseQueryString, slashesDenoteHost) {
   } else if (firstIdx > 0) {
     this.pathname = rest.slice(0, firstIdx);
   }
-  if (slashedProtocol[lowerProto] &&
+  if (slashedProtocol.has(lowerProto) &&
       this.hostname && !this.pathname) {
     this.pathname = '/';
   }
@@ -629,7 +631,7 @@ Url.prototype.format = function format() {
 
   // only the slashedProtocols get the //.  Not mailto:, xmpp:, etc.
   // unless they had them to begin with.
-  if (this.slashes || slashedProtocol[protocol]) {
+  if (this.slashes || slashedProtocol.has(protocol)) {
     if (this.slashes || host) {
       if (pathname && pathname.charCodeAt(0) !== CHAR_FORWARD_SLASH)
         pathname = '/' + pathname;
@@ -701,7 +703,7 @@ Url.prototype.resolveObject = function resolveObject(relative) {
     }
 
     // urlParse appends trailing / to urls like http://www.example.com
-    if (slashedProtocol[result.protocol] &&
+    if (slashedProtocol.has(result.protocol) &&
         result.hostname && !result.pathname) {
       result.path = result.pathname = '/';
     }
@@ -719,7 +721,7 @@ Url.prototype.resolveObject = function resolveObject(relative) {
     // if it is file:, then the host is dropped,
     // because that's known to be hostless.
     // anything else is assumed to be absolute.
-    if (!slashedProtocol[relative.protocol]) {
+    if (!slashedProtocol.has(relative.protocol)) {
       var keys = Object.keys(relative);
       for (var v = 0; v < keys.length; v++) {
         var k = keys[v];
@@ -732,7 +734,7 @@ Url.prototype.resolveObject = function resolveObject(relative) {
     result.protocol = relative.protocol;
     if (!relative.host &&
         !/^file:?$/.test(relative.protocol) &&
-        !hostlessProtocol[relative.protocol]) {
+        !hostlessProtocol.has(relative.protocol)) {
       const relPath = (relative.pathname || '').split('/');
       while (relPath.length && !(relative.host = relPath.shift()));
       if (!relative.host) relative.host = '';
@@ -769,7 +771,8 @@ Url.prototype.resolveObject = function resolveObject(relative) {
   var removeAllDots = mustEndAbs;
   var srcPath = result.pathname && result.pathname.split('/') || [];
   var relPath = relative.pathname && relative.pathname.split('/') || [];
-  var noLeadingSlashes = result.protocol && !slashedProtocol[result.protocol];
+  var noLeadingSlashes = result.protocol &&
+      !slashedProtocol.has(result.protocol);
 
   // if the url is a non-slashed url, then relative
   // links like ../.. should be able