use Object.keys instead of for ... in, because with Opal.

If added some methods to Array.prototype, failed
Blocks.getAllVariableAndListReferences. Because it use for ... in.

src/engine/blocks.js

@@ -799,7 +799,7 @@ class Blocks {
     getAllVariableAndListReferences (optBlocks) {
         const blocks = optBlocks ? optBlocks : this._blocks;
         const allReferences = Object.create(null);
-        for (const blockId in blocks) {
+        Object.keys(blocks).forEach(blockId => {
             let varOrListField = null;
             let varType = null;
             if (blocks[blockId].fields.VARIABLE) {
@@ -823,7 +823,7 @@ class Blocks {
                     }];
                 }
             }
-        }
+        });
         return allReferences;
     }