Add support for "declare interface" Flow syntax

This has been a feature in Flow for a long time (couldn't easily find a
specific commit adding this). Interfaces are basically undocumented, though, so
it's easy to see how this was missed.

src/plugins/flow.js

@@ -60,6 +60,8 @@ pp.flowParseDeclare = function (node) {
     return this.flowParseDeclareModule(node);
   } else if (this.isContextual("type")) {
     return this.flowParseDeclareTypeAlias(node);
+  } else if (this.isContextual("interface")) {
+    return this.flowParseDeclareInterface(node);
   } else {
     this.unexpected();
   }
@@ -104,6 +106,12 @@ pp.flowParseDeclareTypeAlias = function (node) {
   return this.finishNode(node, "DeclareTypeAlias");
 };
 
+pp.flowParseDeclareInterface = function (node) {
+  this.next();
+  this.flowParseInterfaceish(node);
+  return this.finishNode(node, "DeclareInterface");
+}
+
 
 // Interfaces