Actually fix resume token debugging in spec tests (#2877)

Author: schmidt-sebastian

packages/firestore/test/unit/specs/spec_test_runner.ts

@@ -1206,7 +1206,7 @@ abstract class TestRunner {
         `ResumeToken does not match - expected:
          ${stringFromBase64String(
            expectedTarget.resumeToken
-         )}, actual: ${stringFromBase64String(expectedTarget.resumeToken)}`
+         )}, actual: ${stringFromBase64String(actualTarget.resumeToken)}`
       );
       delete actualTargets[targetId];
     });

packages/firestore/test/util/helpers.ts

@@ -524,13 +524,12 @@ export function byteStringFromString(value: string): ByteString {
  */
 export function stringFromBase64String(
   value?: string | Uint8Array
-): ByteString {
+): string {
   assert(
     value === undefined || typeof value === 'string',
     'Can only decode base64 encoded strings'
   );
-  const base64 = PlatformSupport.getPlatform().btoa(value ?? '');
-  return ByteString.fromBase64String(base64);
+  return PlatformSupport.getPlatform().atob(value ?? '');
 }
 
 /** Creates a resume token to match the given snapshot version. */