Make it an error to call close twice on a file using the async API.

tests/standalone/src/io/FileInvalidArgumentsTest.dart

Index: tests/standalone/src/io/FileInvalidArgumentsTest.dart
diff --git a/tests/standalone/src/io/FileInvalidArgumentsTest.dart b/tests/standalone/src/io/FileInvalidArgumentsTest.dart
index 413add9eb9e74f14eba5dd1e44a2891695df34e3..c2add31d588a3c4f0f2d841bba46b96f92e3d155 100644
--- a/tests/standalone/src/io/FileInvalidArgumentsTest.dart
+++ b/tests/standalone/src/io/FileInvalidArgumentsTest.dart
@@ -66,7 +66,8 @@ class FileTest {
     var errors = 0;
     file.onError = (s) {
       errors++;
-      Expect.isTrue(s.contains('Invalid arguments'));
+      Expect.isTrue(s is FileIOException);
+      Expect.isTrue(s.toString().contains('Invalid arguments'));
       file.close(() {
         Expect.equals(1, errors);
       });
@@ -90,7 +91,8 @@ class FileTest {
     var errors = 0;
     file.onError = (s) {
       errors++;
-      Expect.isTrue(s.contains('Invalid argument'));
+      Expect.isTrue(s is FileIOException);
+      Expect.isTrue(s.toString().contains('Invalid argument'));
       file.close(() {
         Expect.equals(1, errors);
       });
@@ -116,7 +118,8 @@ class FileTest {
     var errors = 0;
     file.onError = (s) {
       errors++;
-      Expect.isTrue(s.contains('Invalid arguments'));
+      Expect.isTrue(s is FileIOException);
+      Expect.isTrue(s.toString().contains('Invalid arguments'));
       file.close(() {
         Expect.equals(1, errors);
       });