Add support for mixins to Babel 6.x

src/plugins/flow.js

@@ -109,6 +109,7 @@ pp.flowParseInterfaceish = function (node, allowStatic) {
   }
 
   node.extends = [];
+  node.mixins = [];
 
   if (this.eat(tt._extends)) {
     do {
@@ -116,6 +117,13 @@ pp.flowParseInterfaceish = function (node, allowStatic) {
     } while (this.eat(tt.comma));
   }
 
+  if (this.isContextual("mixins")) {
+    this.next();
+    do {
+      node.mixins.push(this.flowParseInterfaceExtends());
+    } while (this.eat(tt.comma));
+  }
+
   node.body = this.flowParseObjectType(allowStatic);
 };