Re-import set-constant scriptlet from upstream

Base commit:
- gorhill/uBlock@85cf8f5

Related commit:
- gorhill/uBlock@152cea2

assets/resources/resources.txt

@@ -1775,7 +1775,8 @@ disable-newtab-links.js application/javascript
 
 set-constant.js application/javascript
 (function() {
- var cValue = '{{2}}';
+ const thisScript = document.currentScript;
+ let cValue = '{{2}}';
  if ( cValue === 'undefined' ) {
  cValue = undefined;
  } else if ( cValue === 'false' ) {
@@ -1799,26 +1800,27 @@ set-constant.js application/javascript
  } else {
  return;
  }
- var aborted = false;
- var mustAbort = function(v) {
+ let aborted = false;
+ const mustAbort = function(v) {
  if ( aborted ) { return true; }
  aborted =
  (v !== undefined && v !== null) &&
  (cValue !== undefined && cValue !== null) &&
  (typeof v !== typeof cValue);
  return aborted;
  };
- var chain = '{{1}}', owner = window;
- var makeProxy = function(owner, chain) {
- var desc;
- var pos = chain.indexOf('.');
+ const makeProxy = function(owner, chain) {
+ const pos = chain.indexOf('.');
  if ( pos === -1 ) {
- if ( mustAbort(owner[chain]) ) { return; }
- desc = Object.getOwnPropertyDescriptor(owner, chain);
+ const original = owner[chain];
+ if ( mustAbort(original) ) { return; }
+ const desc = Object.getOwnPropertyDescriptor(owner, chain);
  if ( desc === undefined || desc.get === undefined ) {
  Object.defineProperty(owner, chain, {
  get: function() {
- return cValue;
+ return document.currentScript === thisScript
+ ? original
+ : cValue;
  },
  set: function(a) {
  if ( mustAbort(a) ) {
@@ -1829,18 +1831,18 @@ set-constant.js application/javascript
  }
  return;
  }
- var prop = chain.slice(0, pos),
-  v = owner[prop];
- chain = chain.slice(pos + 1); 
+ const prop = chain.slice(0, pos);
+ let v = owner[prop];
+ chain = chain.slice(pos + 1);
  if (
  (v instanceof Object) ||
  (typeof v === 'object' && v !== null)
  ) {
  makeProxy(v, chain);
  return;
  }
- desc = Object.getOwnPropertyDescriptor(owner, prop);
- if ( desc && desc.set ) { return; }
+ const desc = Object.getOwnPropertyDescriptor(owner, prop);
+ if ( desc && desc.set !== undefined ) { return; }
  Object.defineProperty(owner, prop, {
  get: function() {
  return v;
@@ -1853,7 +1855,7 @@ set-constant.js application/javascript
  }
  });
  };
- makeProxy(owner, chain);
+ makeProxy(window, '{{1}}');
 })();