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

tests/standalone/io/file_non_ascii_test.dart

Index: tests/standalone/io/file_non_ascii_test.dart
diff --git a/tests/standalone/io/file_non_ascii_test.dart b/tests/standalone/io/file_non_ascii_test.dart
index 437528ffdc34b789d35f9b5b157727bc5f1c1bd8..d08208069a489f5880f1b687698745de9f061a80 100644
--- a/tests/standalone/io/file_non_ascii_test.dart
+++ b/tests/standalone/io/file_non_ascii_test.dart
@@ -25,7 +25,7 @@ main() {
             f.fullPath().then((p) {
               Expect.isTrue(p.endsWith('${precomposed}.dat') ||
                             p.endsWith('${decomposed}.dat'));
-              f.readAsText().then((contents) {
+              f.readAsString().then((contents) {
                 Expect.equals(precomposed, contents);
                 port.close();
               });