Wrap dart:io exception in getChildren(), implement for the memory FS.

pkg/analyzer/test/file_system/memory_file_system_test.dart

 // Copyright (c) 2014, 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 test.memory_file_system;
 
 import 'dart:async';
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/memory_file_system.dart';
@@ skipping 377 matching lines @@
         });
 
         test('folder', () {
           provider.newFolder('/foo/bar/foldername');
           Folder child = folder.getChildAssumingFolder('foldername');
           expect(child, isNotNull);
           expect(child.exists, isTrue);
         });
       });
 
-      test('getChildren', () {
-        provider.newFile('/foo/bar/a.txt', 'aaa');
-        provider.newFolder('/foo/bar/bFolder');
-        provider.newFile('/foo/bar/c.txt', 'ccc');
-        // prepare 3 children
-        List<Resource> children = folder.getChildren();
-        expect(children, hasLength(3));
-        children.sort((a, b) => a.shortName.compareTo(b.shortName));
-        // check that each child exists
-        children.forEach((child) {
-          expect(child.exists, true);
+      group('getChildren', () {
+        test('exists', () {
+          provider.newFile('/foo/bar/a.txt', 'aaa');
+          provider.newFolder('/foo/bar/bFolder');
+          provider.newFile('/foo/bar/c.txt', 'ccc');
+          // prepare 3 children
+          List<Resource> children = folder.getChildren();
+          expect(children, hasLength(3));
+          children.sort((a, b) => a.shortName.compareTo(b.shortName));
+          // check that each child exists
+          children.forEach((child) {
+            expect(child.exists, true);
+          });
+          // check names
+          expect(children[0].shortName, 'a.txt');
+          expect(children[1].shortName, 'bFolder');
+          expect(children[2].shortName, 'c.txt');
+          // check types
+          expect(children[0], _isFile);
+          expect(children[1], _isFolder);
+          expect(children[2], _isFile);
         });
-        // check names
-        expect(children[0].shortName, 'a.txt');
-        expect(children[1].shortName, 'bFolder');
-        expect(children[2].shortName, 'c.txt');
-        // check types
-        expect(children[0], _isFile);
-        expect(children[1], _isFolder);
-        expect(children[2], _isFile);
+
+        test('does not exist', () {
+          folder = folder.getChildAssumingFolder('no-such-folder');
+          expect(() {
+            folder.getChildren();
+          }, throwsA(_isFileSystemException));
+        });
       });
 
       test('parent', () {
         Resource parent1 = folder.parent;
         expect(parent1, new isInstanceOf<Folder>());
         expect(parent1.path, equals('/foo'));
         Resource parent2 = parent1.parent;
         expect(parent2, new isInstanceOf<Folder>());
         expect(parent2.path, equals('/'));
         expect(parent2.parent, isNull);
@@ skipping 117 matching lines @@
 
         test('resolveRelative', () {
           Uri relative =
               source.resolveRelativeUri(new Uri.file('bar/baz.dart'));
           expect(relative.path, '/foo/bar/baz.dart');
         });
       });
     });
   });
 }