Fix incorrect unusable-by-js warning.

@JsFunction interfaces were not completely loaded by UnifyAst
if the functional method was never called. This fact caused that
the interface was not considered as implementable externally, which
in turn confused restriction checker ending in a spurious warning.

Bug: #9376
Bug-Link: https://github.com/gwtproject/gwt/issues/9376
Change-Id: I92870fde72ab864bc42db8355e283ef3fe17d2ca

dev/core/src/com/google/gwt/dev/jjs/impl/UnifyAst.java

diff --git a/dev/core/src/com/google/gwt/dev/jjs/impl/UnifyAst.java b/dev/core/src/com/google/gwt/dev/jjs/impl/UnifyAst.java
index 702cae1..b09cc19 100644
--- a/dev/core/src/com/google/gwt/dev/jjs/impl/UnifyAst.java
+++ b/dev/core/src/com/google/gwt/dev/jjs/impl/UnifyAst.java
@@ -1039,7 +1039,7 @@
        * that the constructor is considered reachable as it might be needed later for instanceof
        * and casts.
        */
-      if (type.hasJsInteropEntryPoints() || type.isJsNative()) {
+      if (type.hasJsInteropEntryPoints() || type.isJsNative() || type.isJsFunction()) {
         fullFlowIntoType(type);
       }
     }