Rename File.readAsText to File.readAsString. There is no Text type in Dart

tests/standalone/io/file_non_ascii_sync_test.dart

Index: tests/standalone/io/file_non_ascii_sync_test.dart
diff --git a/tests/standalone/io/file_non_ascii_sync_test.dart b/tests/standalone/io/file_non_ascii_sync_test.dart
index 170865229b4e045c1c98c405fa5723ef32972353..3a10b934dadfff6afa9c3ea0cbbbe698aadb3345 100644
--- a/tests/standalone/io/file_non_ascii_sync_test.dart
+++ b/tests/standalone/io/file_non_ascii_sync_test.dart
@@ -21,5 +21,5 @@ main() {
   Expect.isTrue(f.fullPathSync().endsWith('${precomposed}.dat') ||
                 f.fullPathSync().endsWith('${decomposed}.dat'));
   // The contents of the file is precomposed utf8.
-  Expect.equals(precomposed, f.readAsTextSync());
+  Expect.equals(precomposed, f.readAsStringSync());
 }