Use Set instead of Array for validContainers. (reverted from commit 04f939f)

Author: gkellogg

lib/context.js

@@ -382,14 +382,14 @@ api.createTermDefinition = (activeCtx, localCtx, term, defined) => {
   if('@container' in value) {
     // normalize container to an array form
     const container = _isString(value['@container']) ? [value['@container']] : (value['@container'] || []);
-    const validContainers = new Set(['@list', '@set', '@index', '@language']);
+    const validContainers = ['@list', '@set', '@index', '@language'];
     let isValid = true;
     let hasSet = container.includes('@set');
 
     // JSON-LD 1.1 support
     if(activeCtx.processingMode && activeCtx.processingMode.indexOf('json-ld-1.1') === 0) {
       // TODO: @id and @type
-      validContainers.add('@graph');
+      validContainers.push('@graph');
 
       // check container length
       isValid &= container.length <= (hasSet ? 2 : 1);
@@ -402,15 +402,15 @@ api.createTermDefinition = (activeCtx, localCtx, term, defined) => {
     }
 
     // check against valid containers
-    isValid &= container.every(c => validContainers.has(c));
+    isValid &= container.every(c => validContainers.includes(c));
 
     // @set not allowed with @list
     isValid &= !(hasSet && container.includes('@list'));
 
     if(!isValid) {
       throw new JsonLdError(
         'Invalid JSON-LD syntax; @context @container value must be ' +
-        'one of the following: ' + Array.from(validContainers).join(', '),
+        'one of the following: ' + validContainers.join(', '),
         'jsonld.SyntaxError',
         {code: 'invalid container mapping', context: localCtx});
     }