Add tests for unions

tests/unions.js

@@ -0,0 +1,51 @@
+type Type = {
+    a: string | number,
+    b: ?string,
+};
+
+interface Interface {
+    a: string | number;
+    b: ?string;
+}
+
+class Class {
+    a: string | number;
+    b: ?string;
+}
+
+// ###
+({
+    Type: {
+        type: 'record',
+        name: 'Type',
+        fields: [{
+            name: 'a',
+            type: ['string', 'double'],
+        }, {
+            name: 'b',
+            type: ['null', 'string'],
+        }],
+    },
+    Interface: {
+        type: 'record',
+        name: 'Interface',
+        fields: [{
+            name: 'a',
+            type: ['string', 'double'],
+        }, {
+            name: 'b',
+            type: ['null', 'string'],
+        }],
+    },
+    Class: {
+        type: 'record',
+        name: 'Class',
+        fields: [{
+            name: 'a',
+            type: ['string', 'double'],
+        }, {
+            name: 'b',
+            type: ['null', 'string'],
+        }],
+    },
+});