CLJS-1367: Expose default-warning-handler and warning-enabled?

As :warning-handlers option overwrites the handlers collection there
should be a way to use default-warning-handler with :warning-handlers
option.

Warning-enabled? function is useful for custom warning-handlers where it
might be necessary to check if warning was enabled.

Author: Deraen

src/main/clojure/cljs/analyzer.cljc

@@ -339,7 +339,7 @@
   (str "Unsupported JavaScript module type " module-type " for foreign library "
        file "."))
 
-(defn ^:private default-warning-handler [warning-type env extra]
+(defn default-warning-handler [warning-type env extra]
   (when (warning-type *cljs-warnings*)
     (when-let [s (error-message warning-type extra)]
       #?(:clj  (binding [*out* *err*]

src/main/clojure/cljs/analyzer/api.clj

@@ -40,6 +40,18 @@
     `(binding [ana/*cljs-warnings* ~no-warnings]
        ~@body)))
 
+(defn warning-enabled?
+  "Test if the given warning-type is enabled."
+  [warning-type]
+  (ana/*cljs-warnings* warning-type))
+
+(defn default-warning-handler
+  "The default warning handler.
+
+   Outputs the warning messages to *err*."
+  [warning-type env extra]
+  (ana/default-warning-handler warning-type env extra))
+
 (defn get-options
   "Return the compiler options from compiler state."
   ([] (get-options env/*compiler*))

src/test/clojure/cljs/analyzer_api_tests.clj

@@ -2,6 +2,12 @@
   (:require [cljs.analyzer.api :as ana-api])
   (:use clojure.test))
 
+(deftest cljs-warning-test
+  (is (ana-api/warning-enabled? :undeclared-var)
+      "Undeclared-var warning is enabled by default")
+  (is (not (ana-api/no-warn (ana-api/warning-enabled? :undeclared-var)))
+      "Disabled when all warnings are disabled"))
+
 (def warning-form
   '(do (defn x [a b] (+ a b))
        (x 1 2 3 4)))