fix class extends expression (#1956)

lib/output.js

@@ -1474,10 +1474,20 @@ function OutputStream(options) {
             output.space();
         }
         if (self.extends) {
+            var parens = self.extends instanceof AST_Binary ||
+                self.extends instanceof AST_Conditional;
             output.print("extends");
-            output.space();
+            if (parens) {
+                output.print("(");
+            } else {
+                output.space();
+            }
             self.extends.print(output);
-            output.space();
+            if (parens) {
+                output.print(")");
+            } else {
+                output.space();
+            }
         }
         if (self.properties.length > 0) output.with_block(function(){
             self.properties.forEach(function(prop, i){

test/compress/harmony.js

@@ -533,3 +533,15 @@ issue_1753_disable: {
         }
     }
 }
+
+class_extends_expression: {
+    options = {
+        evaluate: true
+    }
+    input: {
+        class bin extends (a || b) {}
+        class seq extends (a, b) {}
+        class ter extends (a ? b : c) {}
+    }
+    expect_exact: "class bin extends(a||b){}class seq extends(a,b){}class ter extends(a?b:c){}"
+}