Add support for Flow's proto modifier syntax (#4551)

Flow commit: https://github.com/facebook/flow/commit/eb815be907eeae9f4851ff154087ebf079510d18

Babel PR: https://github.com/babel/babel/pull/7978

src/language-js/printer-estree.js

@@ -2675,8 +2675,16 @@ function printPathNoParens(path, options, print, args) {
     case "ObjectTypeProperty": {
       const variance = getFlowVariance(n);
 
+      let modifier = "";
+
+      if (n.proto) {
+        modifier = "proto ";
+      } else if (n.static) {
+        modifier = "static ";
+      }
+
       return concat([
-        n.static ? "static " : "",
+        modifier,
         isGetterOrSetter(n) ? n.kind + " " : "",
         variance || "",
         printPropertyKey(path, options, print),

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

@@ -0,0 +1,12 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`proto_props.js 1`] = `
+declare class A { proto: T; }
+declare class B { proto x: T; }
+declare class C { proto +x: T; }
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+declare class A { proto: T }
+declare class B { proto x: T }
+declare class C { proto +x: T }
+
+`;

tests/flow_proto_props/jsfmt.spec.js

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

tests/flow_proto_props/proto_props.js

@@ -0,0 +1,3 @@
+declare class A { proto: T; }
+declare class B { proto x: T; }
+declare class C { proto +x: T; }