Update the input language level of Closure compiler

This allows us to use more modern features like Object spread.

Change-Id: I3a8a119443cf43b27e3ac9390e188408976d82e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1823847Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Tim Van der Lippe <tvanderlippe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699736}

third_party/blink/renderer/devtools/scripts/compile_frontend.py

@@ -245,7 +245,7 @@ common_closure_args = [
     'SIMPLE_OPTIMIZATIONS',
     '--warning_level',
     'VERBOSE',
-    '--language_in=ECMASCRIPT_2017',
+    '--language_in=ECMASCRIPT_NEXT',
     '--language_out=ES5_STRICT',
     '--extra_annotation_name',
     'suppressReceiverCheck',