Fixup from comments

src/serialization/sb3.js

@@ -274,7 +274,7 @@ const compressInputTree = function (block, blocks) {
 /**
  * Get non-core extension ID for a given sb3 opcode.
  * @param {!string} opcode The opcode to examine for extension.
- * @return {?string} The extension ID, if it exists/is not a core extension.
+ * @return {?string} The extension ID, if it exists and is not a core extension.
  */
 const getExtensionIdForOpcode = function (opcode) {
     const index = opcode.indexOf('_');

test/unit/serialization_sb3.js

@@ -230,5 +230,9 @@ test('getExtensionIdForOpcode', t => {
 
     // only considers things before the first underscore
     t.equal(sb3.getExtensionIdForOpcode('hello_there_loopy'), 'hello');
+
+    // does not return anything for opcodes with no extension
+    t.false(sb3.getExtensionIdForOpcode('hello'));
+
     t.end();
 });