Merge pull request #1011 from kzc/dont-produce-let-in-mangle

Do not produce `let` as a variable name in mangle.

lib/parse.js

@@ -46,7 +46,7 @@
 
 var KEYWORDS = 'break case catch const continue debugger default delete do else finally for function if in instanceof new return switch throw try typeof var void while with';
 var KEYWORDS_ATOM = 'false null true';
-var RESERVED_WORDS = 'abstract boolean byte char class double enum export extends final float goto implements import int interface long native package private protected public short static super synchronized this throws transient volatile yield'
+var RESERVED_WORDS = 'abstract boolean byte char class double enum export extends final float goto implements import int interface let long native package private protected public short static super synchronized this throws transient volatile yield'
     + " " + KEYWORDS_ATOM + " " + KEYWORDS;
 var KEYWORDS_BEFORE_EXPRESSION = 'return new delete throw else case';