add test case for passing in $data as prop

Author: yyx990803

src/instance/scope.js

@@ -31,8 +31,8 @@ exports._initData = function () {
     i = props.length
     while (i--) {
       key = _.camelize(props[i])
-      if (!(key in data)) {
-        data[key] = null
+      if (!(key in data) && key !== '$data') {
+        data[key] = undefined
       }
     }
   }
@@ -65,7 +65,9 @@ exports._setData = function (newData) {
     i = props.length
     while (i--) {
       key = props[i]
-      newData.$set(key, oldData[key])
+      if (key !== '$data') {
+        newData.$set(key, oldData[key])
+      }
     }
   }
   // unproxy keys not present in new data

test/unit/specs/directives/prop_spec.js

@@ -58,6 +58,30 @@ if (_.inBrowser) {
       })
     })
 
+    it('$data as prop', function (done) {
+      var vm = new Vue({
+        el: el,
+        template: '<test $data="{{ok}}"></test>',
+        data: {
+          ok: {
+            msg: 'hihi'
+          }
+        },
+        components: {
+          test: {
+            props: ['$data'],
+            template: '{{msg}}'
+          }
+        }
+      })
+      expect(el.innerHTML).toBe('<test>hihi</test>')
+      vm.ok = { msg: 'what' }
+      _.nextTick(function () {
+        expect(el.innerHTML).toBe('<test>what</test>')
+        done()
+      })
+    })
+
     it('warn invalid keys', function () {
       var vm = new Vue({
         el: el,