Patch Closure Compiler to fix polyfill+wrapper

When using a module wrapper, polyfills inserted by the compiler do not
go to "window", so all internal references to "Symbol" from those
polyfills must go through "$jscomp.global".  This patches the compiler
to fix this issue.

Upstream: google/closure-compiler#2957
Closes shaka-project#1455

Change-Id: Ifb9a4f651f31c3099d660c4b818da8457d89dbbc

third_party/SUMMARY.txt

@@ -1,12 +1,10 @@
 closure/compiler.jar
-  The closure compiler, v20171203, by Google.
-  Apache v2.0 license.
-  https://github.com/google/closure-compiler
-
-closure/linter.jar
-  The closure compiler linter, v20171203, by Google.
+  The closure compiler, v20180402 (modified), by Google.
   Apache v2.0 license.
   https://github.com/google/closure-compiler
+  Local mods:
+    - Fixed issues with Symbol polyfill interactions with output wrappers
+      - https://github.com/google/closure-compiler/issues/2957
 
 closure/goog
   The closure library (modified), possibly v20150315, by Google.