Make bzl files compatible with mutating behavior of += on lists

In the future versions of Bazel (starting with 0.7 or 0.8) += on lists
will behave like in Python, by mutating the left hand side list. If this
list is frozen it will cause a runtime error. To preserve the previous
behavior such lists should be either copied or the `a = a + b` syntax
should be used (which is completely equivalent with the old behavior of
+= on lists).

Change-Id: I3ef62e0c1f3b5ceca705ed4fa331122c3190991e

tools/bzl/gwt.bzl

@@ -98,7 +98,7 @@ USER_AGENT_XML = """<module rename-to='gerrit_ui'>
 
 def gwt_module(gwt_xml=None, resources=[], srcs=[], **kwargs):
   if gwt_xml:
-    resources += [gwt_xml]
+    resources = resources + [gwt_xml]
 
   java_library2(
     srcs = srcs,

tools/bzl/java.bzl

@@ -17,8 +17,8 @@
 
 def java_library2(deps=[], exported_deps=[], exports=[], **kwargs):
   if exported_deps:
-    deps += exported_deps
-    exports += exported_deps
+    deps = deps + exported_deps
+    exports = exports + exported_deps
   native.java_library(
     deps = deps,
     exports = exports,