Support alternative syntax for exact object

Sometimes `{ [any]: empty }` is used as a workaround instead of `{||}`.

See:
- facebook/flow#2977
- facebook/flow#4582

packages/babel-plugin-flow-runtime/src/__tests__/__fixtures__/typeAliases/objects/objectWithExactIndexer.js

@@ -0,0 +1,15 @@
+/* @flow */
+
+export const input = `
+  type Demo = {
+    foo?: number;
+    [any]: empty;
+  };
+`;
+
+export const expected = `
+  import t from "flow-runtime";
+  const Demo = t.type("Demo", t.exactObject(
+    t.property("foo", t.number(), true)
+  ));
+`;

packages/babel-plugin-flow-runtime/src/convert.js

@@ -762,12 +762,21 @@ converters.ObjectTypeAnnotation = (context: ConversionContext, path: NodePath):
     [[], new Map(), new Map()]
   );
 
+  const isEmptyObjectTypeIndexer = (path: NodePath) => path.node.type === 'ObjectTypeIndexer'
+    && path.node.key.type === 'AnyTypeAnnotation'
+    && path.node.value.type === 'EmptyTypeAnnotation';
+
+  const indexers = path.get('indexers');
+
   const body = [
     ...path.get('callProperties'),
     ...properties,
-    ...path.get('indexers')
+    ...indexers.filter(path => !isEmptyObjectTypeIndexer(path))
   ];
-  return context.call(path.node.exact ? 'exactObject' : 'object', ...body.map(item => convert(context, item)));
+
+  const isExactObject = path.node.exact || indexers.some(path => isEmptyObjectTypeIndexer(path));
+
+  return context.call(isExactObject ? 'exactObject' : 'object', ...body.map(item => convert(context, item)));
 };
 
 converters.ObjectTypeSpreadProperty = (context: ConversionContext, path: NodePath): Node => {