Ignore trailing comma from the ast (#1761)

This doesn't change the semantics

src/clean-ast.js

@@ -48,7 +48,8 @@ function massageAST(ast) {
       "id",
       "source",
       "before",
-      "after"
+      "after",
+      "trailingComma"
     ].forEach(name => {
       delete newObj[name];
     });

tests/css_trailing_comma/__snapshots__/jsfmt.spec.js.snap

@@ -0,0 +1,13 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`trailing.css 1`] = `
+asdasldaskdhjkashdahsdkjahskdjhakjsdkjahsdhkasdhkajsdhakjsdhkajsdhjkahskjdkjahsjkdjkakjsdm,
+asdasldaskdhjkashdahsdkjahskdjhakjsdkjahsdhkasdhkajsdhakjsdhkajsdhjkahskjdkjahsjkdjkakjsdm, {
+
+}
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+asdasldaskdhjkashdahsdkjahskdjhakjsdkjahsdhkasdhkajsdhakjsdhkajsdhjkahskjdkjahsjkdjkakjsdm,
+asdasldaskdhjkashdahsdkjahskdjhakjsdkjahsdhkasdhkajsdhakjsdhkajsdhjkahskjdkjahsjkdjkakjsdm {
+}
+
+`;

tests/css_trailing_comma/jsfmt.spec.js

@@ -0,0 +1 @@
+run_spec(__dirname, { parser: "postcss" });

tests/css_trailing_comma/trailing.css

@@ -0,0 +1,4 @@
+asdasldaskdhjkashdahsdkjahskdjhakjsdkjahsdhkasdhkajsdhakjsdhkajsdhjkahskjdkjahsjkdjkakjsdm,
+asdasldaskdhjkashdahsdkjahskdjhakjsdkjahsdhkasdhkajsdhakjsdhkajsdhjkahskjdkjahsjkdjkakjsdm, {
+
+}