Use load instead of require to load core cljs macros

Loading now happens dynamically and once when the
analyzer is first ran.
Also provides macros to facilitate overriding of the
default path provided (defaults to cljs/core).

src/clj/cljs/compiler.clj

@@ -22,7 +22,6 @@
 (declare confirm-bindings)
 (declare munge)
 (declare ^:dynamic *cljs-file*)
-(require 'cljs.core)
 
 (def js-reserved
   #{"abstract" "boolean" "break" "byte" "case"
@@ -63,6 +62,33 @@
 (def ^:dynamic *unchecked-if* (atom false))
 (def ^:dynamic *cljs-static-fns* false)
 (def ^:dynamic *position* nil)
+(def ^:dynamic *cljs-macros-path* "/cljs/core")
+(def ^:dynamic *cljs-macros-is-classpath* true)
+(def  -cljs-macros-loaded (atom false))
+
+(defn load-core []
+  (when (not @-cljs-macros-loaded)
+    (reset! -cljs-macros-loaded true)
+    (if *cljs-macros-is-classpath*
+      (load *cljs-macros-path*)
+      (load-file *cljs-macros-path*))))
+
+(defmacro with-core-macros
+  [path & body]
+  `(do
+     (when (not= *cljs-macros-path* ~path)
+       (reset! -cljs-macros-loaded false))
+     (binding [*cljs-macros-path* ~path]
+       ~@body)))
+
+(defmacro with-core-macros-file
+  [path & body]
+  `(do
+     (when (not= *cljs-macros-path* ~path)
+       (reset! -cljs-macros-loaded false))
+     (binding [*cljs-macros-path* ~path
+               *cljs-macros-is-classpath* false]
+       ~@body)))
 
 (defn empty-env []
   {:ns (@namespaces *cljs-ns*) :context :statement :locals {}})
@@ -1308,7 +1334,7 @@
     (when (seq @deps)
       (analyze-deps @deps))
     (set! *cljs-ns* name)
-    (require 'cljs.core)
+    (load-core)
     (doseq [nsym (concat (vals requires-macros) (vals uses-macros))]
       (clojure.core/require nsym))
     (swap! namespaces #(-> %
@@ -1581,6 +1607,7 @@
      (let [form (if (instance? clojure.lang.LazySeq form)
                   (or (seq form) ())
                   form)]
+       (load-core)
        (cond
         (symbol? form) (analyze-symbol env form)
         (and (seq? form) (seq form)) (analyze-seq env form name)