fix(dart/transform): Handle hostAttributes in DirectiveMetadata

Handle `hostAttributes` in the transformer.
`hostAttributes` was introduced in 51839ca677

Closes #1742

modules/angular2/test/transform/integration/simple_annotation_files/expected/bar.ng_meta.json

@@ -1 +1,14 @@
-{"MyComponent":{"id":"MyComponent","selector":"[soup]","compileChildren":true,"hostListeners":{},"hostProperties":{},"properties":{},"readAttributes":[],"type":1,"version":1}}
+{
+  "MyComponent": {
+    "id": "MyComponent",
+    "selector": "[soup]",
+    "compileChildren": true,
+    "hostListeners": {},
+    "hostProperties": {},
+    "hostAttributes": {},
+    "properties": {},
+    "readAttributes": [],
+    "type": 1,
+    "version": 1
+  }
+}