Throw FileSystemException from FileSystemEntity methods.

pkg/front_end/lib/memory_file_system.dart

 // Copyright (c) 2016, 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.
 
 library front_end.memory_file_system;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:typed_data';
 
@@ skipping 49 matching lines @@
   @override
   int get hashCode => uri.hashCode;
 
   @override
   bool operator ==(Object other) =>
       other is MemoryFileSystemEntity &&
       other.uri == uri &&
       identical(other._fileSystem, _fileSystem);
 
   @override
-  Future<List<int>> readAsBytes() async {
-    List<int> contents = _fileSystem._files[uri];
-    if (contents != null) {
-      return contents.toList();
-    }
-    throw new Exception('File does not exist');
-  }
-
-  @override
-  Future<String> readAsString() async {
-    List<int> contents = await readAsBytes();
-    return UTF8.decode(contents);
-  }
-
-  @override
   Future<bool> exists() async => _fileSystem._files[uri] != null;
 
   @override
   Future<DateTime> lastModified() async {
     var lastModified = _fileSystem._lastModified[uri];
     if (lastModified == null) {
-      throw new Exception('File does not exist');
+      throw new FileSystemException(uri, 'File $uri does not exist.');
     }
     return lastModified;
   }
 
+  @override
+  Future<List<int>> readAsBytes() async {
+    List<int> contents = _fileSystem._files[uri];
+    if (contents == null) {
+      throw new FileSystemException(uri, 'File $uri does not exist.');
+    }
+    return contents.toList();
+  }
+
+  @override
+  Future<String> readAsString() async {
+    List<int> bytes = await readAsBytes();
+    try {
+      return UTF8.decode(bytes);
+    } on FormatException catch (e) {
+      throw new FileSystemException(uri, e.message);
+    }
+  }
+
   /// Writes the given raw bytes to this file system entity.
   ///
   /// If no file exists, one is created.  If a file exists already, it is
   /// overwritten.
   void writeAsBytesSync(List<int> bytes) {
     _update(uri, new Uint8List.fromList(bytes));
   }
 
   /// Writes the given string to this file system entity.
   ///
   /// The string is encoded as UTF-8.
   ///
   /// If no file exists, one is created.  If a file exists already, it is
   /// overwritten.
   void writeAsStringSync(String s) {
     // Note: the return type of UTF8.encode is List<int>, but in practice it
     // always returns Uint8List.  We rely on that for efficiency, so that we
     // don't have to make an extra copy.
     _update(uri, UTF8.encode(s) as Uint8List);
   }
 
   void _update(Uri uri, Uint8List data) {
     _fileSystem._files[uri] = data;
     _fileSystem._lastModified[uri] =
         new DateTime.fromMicrosecondsSinceEpoch(++_fileSystem._lastUpdate);
   }
 }