gwt.bzl: Avoid using deprecated depset union

Change-Id: If01f61085c9a9228e58101f941c4952590c8ad28
diff --git a/tools/bzl/gwt.bzl b/tools/bzl/gwt.bzl
index 4f54d73..467e949 100644
--- a/tools/bzl/gwt.bzl
+++ b/tools/bzl/gwt.bzl
@@ -200,17 +200,17 @@
     )
 
 def _get_transitive_closure(ctx):
-    deps = depset()
+    deps = []
     for dep in ctx.attr.module_deps:
-        deps += dep.java.transitive_runtime_deps
-        deps += dep.java.transitive_source_jars
+        deps.append(dep.java.transitive_runtime_deps)
+        deps.append(dep.java.transitive_source_jars)
     for dep in ctx.attr.deps:
         if hasattr(dep, "java"):
-            deps += dep.java.transitive_runtime_deps
+            deps.append(dep.java.transitive_runtime_deps)
         elif hasattr(dep, "files"):
-            deps += dep.files
+            deps.append(dep.files)
 
-    return deps
+    return depset(transitive = deps)
 
 gwt_binary = rule(
     attrs = {