add support for this in computed property names

test/fixtures/computed-property-names/this/actual.js

@@ -0,0 +1,3 @@
+var obj = {
+  ["x" + this.foo]: "heh"
+};

test/fixtures/computed-property-names/this/expected.js

@@ -0,0 +1,4 @@
+var obj = function (obj) {
+  obj["x" + this.foo] = "heh";
+  return obj;
+}.call(this, {});