add file watching support

pkg/analyzer/lib/file_system/memory_file_system.dart

Index: pkg/analyzer/lib/file_system/memory_file_system.dart
diff --git a/pkg/analyzer/lib/file_system/memory_file_system.dart b/pkg/analyzer/lib/file_system/memory_file_system.dart
index 8d1f1144b732ccb8774f601036047fd91fd8e3ba..d85a224da8aad03c23f9965f05269e79ad956e9f 100644
--- a/pkg/analyzer/lib/file_system/memory_file_system.dart
+++ b/pkg/analyzer/lib/file_system/memory_file_system.dart
@@ -183,6 +183,11 @@ class _MemoryDummyLink extends _MemoryResource implements File {
       : super(provider, path);
 
   @override
+  Stream<WatchEvent> get changes {
+    throw new FileSystemException(path, "File does not exist");
+  }
+
+  @override
   bool get exists => false;
 
   int get modificationStamp {
@@ -358,23 +363,6 @@ class _MemoryFolder extends _MemoryResource implements Folder {
       : super(provider, path);
 
   @override
-  Stream<WatchEvent> get changes {
-    StreamController<WatchEvent> streamController =
-        new StreamController<WatchEvent>();
-    if (!_provider._pathToWatchers.containsKey(path)) {
-      _provider._pathToWatchers[path] = <StreamController<WatchEvent>>[];
-    }
-    _provider._pathToWatchers[path].add(streamController);
-    streamController.done.then((_) {
-      _provider._pathToWatchers[path].remove(streamController);
-      if (_provider._pathToWatchers[path].isEmpty) {
-        _provider._pathToWatchers.remove(path);
-      }
-    });
-    return streamController.stream;
-  }
-
-  @override
   bool get exists => _provider._pathToResource[path] is _MemoryFolder;
 
   @override
@@ -442,6 +430,22 @@ abstract class _MemoryResource implements Resource {
 
   _MemoryResource(this._provider, this.path);
 
+  Stream<WatchEvent> get changes {
+    StreamController<WatchEvent> streamController =
+        new StreamController<WatchEvent>();
+    if (!_provider._pathToWatchers.containsKey(path)) {
+      _provider._pathToWatchers[path] = <StreamController<WatchEvent>>[];
+    }
+    _provider._pathToWatchers[path].add(streamController);
+    streamController.done.then((_) {
+      _provider._pathToWatchers[path].remove(streamController);
+      if (_provider._pathToWatchers[path].isEmpty) {
+        _provider._pathToWatchers.remove(path);
+      }
+    });
+    return streamController.stream;
+  }
+
   @override
   get hashCode => path.hashCode;