More work towards proper testing (flutter#16)

- Add stub tests for Directory. Please review for coverage;
  impl will come later
- Rename tests to follow common pattern
- Fix some cases where we weren't matching the behavior of
  dart:io libraries

Author: tvolkert

lib/src/backends/memory/memory_directory.dart

@@ -48,6 +48,7 @@ class _MemoryDirectory extends _MemoryFileSystemEntity implements Directory {
     String basename = fileSystem._context.basename(fullPath);
     _DirectoryNode node = fileSystem._findNode(dirname);
     _checkExists(node, () => dirname);
+    _checkIsDir(node, () => dirname);
     var name = () => '$basename$_tempCounter';
     while (node.children.containsKey(name())) {
       _tempCounter++;

lib/src/backends/memory/memory_file_system.dart

@@ -99,8 +99,10 @@ class MemoryFileSystem extends FileSystem {
 
   @override
   bool identicalSync(String path1, String path2) {
-    _Node node1 = _findNode(path1, resolveTailLink: true);
-    _Node node2 = _findNode(path2, resolveTailLink: true);
+    _Node node1 = _findNode(path1);
+    _checkExists(node1, () => path1);
+    _Node node2 = _findNode(path2);
+    _checkExists(node2, () => path2);
     return node1 != null && node1 == node2;
   }
 

lib/src/backends/memory/node.dart

@@ -134,8 +134,7 @@ class _FileNode extends _RealNode {
   int get size => content.length;
 
   void copyFrom(_FileNode source) {
-    changed = new DateTime.now().millisecondsSinceEpoch;
-    modified = new DateTime.now().millisecondsSinceEpoch;
+    modified = changed = new DateTime.now().millisecondsSinceEpoch;
     accessed = source.accessed;
     mode = source.mode;
     content = new List<int>.from(content);