🐛Remove string.trimStart() from expander (ampproject#23439)

* use new trimStart

* oops

src/service/url-expander/expander.js

@@ -16,6 +16,7 @@
 
 import {hasOwn} from '../../utils/object';
 import {rethrowAsync, user, userAssert} from '../../log';
+import {trimStart} from '../../string';
 import {tryResolve} from '../../utils/promise';
 
 /** @private @const {string} */
@@ -150,7 +151,7 @@ export class Expander {
           // Collect any chars that may be prefixing the macro, if we are in
           // a nested context trim the args.
           if (builder.trim().length) {
-            results.push(numOfPendingCalls ? builder.trimStart() : builder);
+            results.push(numOfPendingCalls ? trimStart(builder) : builder);
           }
 
           // If we know we are at the start of a macro, we figure out how to

src/string.js

@@ -158,3 +158,16 @@ export function trimEnd(str) {
 
   return ('_' + str).trim().slice(1);
 }
+
+/**
+ * Trims any leading whitespace from a string.
+ * @param {string} str  A string to trim.
+ * @return {string} The string, with leading whitespace removed.
+ */
+export function trimStart(str) {
+  if (str.trimStart) {
+    return str.trimStart();
+  }
+
+  return (str + '_').trim().slice(0, -1);
+}