New source found from dndbeyond.com

ddb_main/node_modules/symbol-observable/es/index.js

@@ -0,0 +1,19 @@
+/* global window */
+import ponyfill from './ponyfill.js';
+
+var root;
+
+if (typeof self !== 'undefined') {
+  root = self;
+} else if (typeof window !== 'undefined') {
+  root = window;
+} else if (typeof global !== 'undefined') {
+  root = global;
+} else if (typeof module !== 'undefined') {
+  root = module;
+} else {
+  root = Function('return this')();
+}
+
+var result = ponyfill(root);
+export default result;

ddb_main/node_modules/symbol-observable/es/ponyfill.js

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+export default function symbolObservablePonyfill(root) {
+	var result;
+	var Symbol = root.Symbol;
+
+	if (typeof Symbol === 'function') {
+		if (Symbol.observable) {
+			result = Symbol.observable;
+		} else {
+			result = Symbol('observable');
+			Symbol.observable = result;
+		}
+	} else {
+		result = '@@observable';
+	}
+
+	return result;
+};