add Safari workaround for await (#2528)

fixes #2344

lib/output.js

@@ -68,6 +68,7 @@ function OutputStream(options) {
         preserve_line    : false,
         quote_keys       : false,
         quote_style      : 0,
+        safari10         : false,
         semicolons       : true,
         shebang          : true,
         shorthand        : undefined,
@@ -677,7 +678,8 @@ function OutputStream(options) {
     PARENS(AST_Await, function(output){
         var p = output.parent();
         return p instanceof AST_PropAccess && p.expression === this
-            || p instanceof AST_Call && p.expression === this;
+            || p instanceof AST_Call && p.expression === this
+            || output.option("safari10") && p instanceof AST_UnaryPrefix;
     });
 
     PARENS(AST_Sequence, function(output){