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){

test/compress/async.js

@@ -293,3 +293,31 @@ async_arrow_iife_negate_iife: {
     }
     expect_exact: "(async()=>{await fetch()})();(()=>{plain()})();"
 }
+
+issue_2344_1: {
+    beautify = {
+        safari10: false,
+    }
+    input: {
+        async () => {
+            +await x;
+            await y;
+            return await z;
+        };
+    }
+    expect_exact: "async()=>{+await x;await y;return await z};"
+}
+
+issue_2344_2: {
+    beautify = {
+        safari10: true,
+    }
+    input: {
+        async () => {
+            +await x;
+            await y;
+            return await z;
+        };
+    }
+    expect_exact: "async()=>{+(await x);await y;return await z};"
+}