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

sdk/lib/io/file_impl.dart

 // Copyright (c) 2013, 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.
 
 part of dart.io;
 
 // Read the file in blocks of size 64k.
 const int _BLOCK_SIZE = 64 * 1024;
 
 
@@ skipping 491 matching lines @@
     var opened = openSync();
     var builder = new BytesBuilder();
     var data;
     while ((data = opened.readSync(_BLOCK_SIZE)).length > 0) {
       builder.add(data);
     }
     opened.closeSync();
     return builder.takeBytes();
   }
 
+  String _tryDecode(List<int> bytes, Encoding encoding) {
+    try {
+      return encoding.decode(bytes);
+    } catch (_) {
+      throw new FileException(
+          "Failed to decode data using encoding '${encoding.name}'", path);
+    }
+  }
+
   Future<String> readAsString({Encoding encoding: UTF8}) {
     _ensureFileService();
     return readAsBytes().then((bytes) {
-      return encoding.decode(bytes);
+      return _tryDecode(bytes, encoding);
     });
   }
 
   String readAsStringSync({Encoding encoding: UTF8}) {
     List<int> bytes = readAsBytesSync();
-    return encoding.decode(bytes);
+    return _tryDecode(bytes, encoding);
   }
 
-  static List<String> _decodeLines(List<int> bytes, Encoding encoding) {
+  List<String> _decodeLines(List<int> bytes, Encoding encoding) {
     if (bytes.length == 0) return [];
     var list = [];
     var controller = new StreamController(sync: true);
+    var error = null;
     controller.stream
         .transform(encoding.decoder)
         .transform(new LineSplitter())
-        .listen((line) => list.add(line));
+        .listen((line) => list.add(line), onError: (e) => error = e);
     controller.add(bytes);
     controller.close();
+    if (error != null) {
+      throw new FileException(
+          "Failed to decode data using encoding '${encoding.name}'", path);
+    }
     return list;
   }
 
   Future<List<String>> readAsLines({Encoding encoding: UTF8}) {
     _ensureFileService();
     return readAsBytes().then((bytes) {
       return _decodeLines(bytes, encoding);
     });
   }
 
@@ skipping 439 matching lines @@
   void _checkNotClosed() {
     if (closed) {
       throw new FileException("File closed", path);
     }
   }
 
   Future _closedException() {
     return new Future.error(new FileException("File closed", path));
   }
 }