Add test for flow class field override (#1669)

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

@@ -0,0 +1,12 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`override.js 1`] = `
+class Foo {
+  constructor: () => this;
+}
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+class Foo {
+  constructor: () => this;
+}
+
+`;

tests/flow_class_field/jsfmt.spec.js

@@ -0,0 +1 @@
+run_spec(__dirname, null, ["babylon"]);

tests/flow_class_field/override.js

@@ -0,0 +1,3 @@
+class Foo {
+  constructor: () => this;
+}