CLJS-2027: Add language-in for ECMA 2017 and ECMA Next

Also upgrades Google Closure Compiler to the May 2017 release which contains
the `CompilerOptions$LanguageMode/ECMASCRIPT_2017` enum.

Author: anmonteiro

Diff for: pom.template.xml

@@ -30,7 +30,7 @@
         <dependency>
             <groupId>com.google.javascript</groupId>
             <artifactId>closure-compiler-unshaded</artifactId>
-            <version>v20170423</version>
+            <version>v20170521</version>
         </dependency>
         <dependency>
             <groupId>org.clojure</groupId>

Diff for: project.clj

@@ -14,7 +14,7 @@
                  [org.clojure/test.check "0.9.0" :scope "test"]
                  [com.cognitect/transit-clj "0.8.300"]
                  [org.clojure/google-closure-library "0.0-20170519-fa0499ef"]
-                 [com.google.javascript/closure-compiler-unshaded "v20170423"]
+                 [com.google.javascript/closure-compiler-unshaded "v20170521"]
                  [org.mozilla/rhino "1.7R5"]]
   :profiles {:1.6 {:dependencies [[org.clojure/clojure "1.6.0"]]}
              :uberjar {:aot :all :main clojure.main}}

Diff for: script/bootstrap

@@ -5,7 +5,7 @@ set -e
 CLOJURE_RELEASE="1.9.0-alpha16"
 SPEC_ALPHA_RELEASE="0.1.108"
 CORE_SPECS_ALPHA_RELEASE="0.1.10"
-CLOSURE_RELEASE="20170423"
+CLOSURE_RELEASE="20170521"
 DJSON_RELEASE="0.2.6"
 TRANSIT_RELEASE="0.8.285"
 GCLOSURE_LIB_RELEASE="0.0-20160609-f42b4a24"

Diff for: src/main/clojure/cljs/closure.clj

@@ -193,6 +193,8 @@
   (case key
     :no-transpile                     CompilerOptions$LanguageMode/NO_TRANSPILE
     (:ecmascript6 :es6)               CompilerOptions$LanguageMode/ECMASCRIPT6
+    (:ecmascript-2017 :es-2017)       CompilerOptions$LanguageMode/ECMASCRIPT_2017
+    (:ecmascript-next :es-next)       CompilerOptions$LanguageMode/ECMASCRIPT_NEXT
     (:ecmascript6-strict :es6-strict) CompilerOptions$LanguageMode/ECMASCRIPT6_STRICT
     (:ecmascript6-typed :es6-typed)   CompilerOptions$LanguageMode/ECMASCRIPT6_TYPED
     (:ecmascript5 :es5)               CompilerOptions$LanguageMode/ECMASCRIPT5