CLJS-646: single segment namespaces and reify don't work

the exists? call in reify was not correct, it was a property access on
the namespace, which would of course get resolved by the compiler,
creating a symbol foo/foo (in the case where foo is the single segment
namespace). now we actually emit a symbol of the namespace and the type
symbol, the exists? macro now annotates this symbol with
:cljs.analyzer/no-resolve, the analyzer now respects this so the user
won't get spurious warnings from using exists? on vars that don't exist.

src/clj/cljs/analyzer.clj

@@ -247,7 +247,9 @@
                      :ns full-ns})))))))
 
 (defn resolve-existing-var [env sym]
-  (resolve-var env sym confirm-var-exists))
+  (if-not (-> sym meta ::no-resolve)
+    (resolve-var env sym confirm-var-exists)
+    (resolve-var env sym)))
 
 (defn confirm-bindings [env names]
   (doseq [name names]

src/clj/cljs/core.clj

@@ -279,8 +279,11 @@
 (defmacro string? [x]
   (bool-expr (list 'js* "typeof ~{} === 'string'" x)))
 
+;; TODO: x must be a symbol, not an arbitrary expression
 (defmacro exists? [x]
-  (bool-expr (list 'js* "typeof ~{} !== 'undefined'" x)))
+  (bool-expr
+    (list 'js* "typeof ~{} !== 'undefined'"
+      (vary-meta x assoc :cljs.analyzer/no-resolve true))))
 
 (defmacro undefined? [x]
   (bool-expr (list 'js* "(void 0 === ~{})" x)))
@@ -576,7 +579,7 @@
         ns     (-> &env :ns :name)
         munge  cljs.compiler/munge]
     `(do
-       (when-not (exists? (. ~ns ~(symbol (core/str "-" t))))
+       (when-not (exists? ~(symbol (core/str ns) (core/str t)))
          (deftype ~t [~@locals ~meta-sym]
            IWithMeta
            (~'-with-meta [~this-sym ~meta-sym]