strip flow directives in flow transformer - fixes #1934

src/babel/transformation/transformers/other/flow.js

@@ -5,6 +5,13 @@ export var metadata = {
 };
 
 export var visitor = {
+  Program(node, parent, scope, file) {
+    for (var comment of (file.ast.comments: Array)) {
+      if (comment.value.indexOf("@flow") >= 0) {
+        comment._displayed = true;
+      }
+    }
+  },
   Flow() {
     this.dangerouslyRemove();
   },

test/core/fixtures/transformation/flow/strip-directive/actual.js

@@ -0,0 +1,9 @@
+/*@flow*/
+
+/* @flow */
+
+/**
+ * @flow
+ */
+
+foo();

test/core/fixtures/transformation/flow/strip-directive/expected.js

@@ -0,0 +1 @@
+foo();