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

pkg/analyzer/test/file_system/memory_file_system_test.dart

Index: pkg/analyzer/test/file_system/memory_file_system_test.dart
diff --git a/pkg/analyzer/test/file_system/memory_file_system_test.dart b/pkg/analyzer/test/file_system/memory_file_system_test.dart
index b17d39ef757949fc456c66b0bfc8c26d638d61c9..457afe3844d430d061373c3062c7462691f48e65 100644
--- a/pkg/analyzer/test/file_system/memory_file_system_test.dart
+++ b/pkg/analyzer/test/file_system/memory_file_system_test.dart
@@ -395,26 +395,35 @@ main() {
         });
       });
 
-      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);
-        });
-        // 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);
+      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);
+        });
+
+        test('does not exist', () {
+          folder = folder.getChildAssumingFolder('no-such-folder');
+          expect(() {
+            folder.getChildren();
+          }, throwsA(_isFileSystemException));
+        });
       });
 
       test('parent', () {