Throw FileExceptions when in File when reading as String and decoding fails.

tests/standalone/io/file_read_encoded_test.dart

+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import "package:expect/expect.dart";
+import "package:async_helper/async_helper.dart";
+import 'dart:io';
+
+void testReadAsString() {
+  var tmp = new Directory('').createTempSync();
+
+  var file = new File('${tmp.path}/file');
+  file.createSync();
+
+  file.writeAsBytesSync([0xb0]);
+
+  Expect.throws(file.readAsStringSync, (e) => e is FileException);
+
+  asyncStart();
+  file.readAsString().then((_) {
+    Expect.fail("expected exception");
+  }).catchError((e) {
+    tmp.deleteSync(recursive: true);
+    asyncEnd();
+  }, test: (e) => e is FileException);
+}
+
+void testReadAsLines() {
+  var tmp = new Directory('').createTempSync();
+
+  var file = new File('${tmp.path}/file');
+  file.createSync();
+
+  file.writeAsBytesSync([0xb0]);
+
+  Expect.throws(file.readAsLinesSync, (e) => e is FileException);
+
+  asyncStart();
+  file.readAsLines().then((_) {
+    Expect.fail("expected exception");
+  }).catchError((e) {
+    tmp.deleteSync(recursive: true);
+    asyncEnd();
+  }, test: (e) => e is FileException);
+}
+
+void main() {
+  testReadAsString();
+  testReadAsLines();
+}