Fix class inside of binary expression missing parenthesis (#397)

Fixes #395

tests/classes/__snapshots__/jsfmt.spec.js.snap

@@ -74,6 +74,19 @@ new E().x;
 "
 `;
 
+exports[`test binary.js 1`] = `
+"(class {}) + 1;
+(class a {}) + 1;
+(class extends b {}) + 1;
+(class a extends b {}) + 1;
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+(class {}) + 1;
+(class a {}) + 1;
+(class extends b {}) + 1;
+(class a extends b {}) + 1;
+"
+`;
+
 exports[`test class_shapes.js 1`] = `
 "/* @flow */
 

tests/classes/binary.js

@@ -0,0 +1,4 @@
+(class {}) + 1;
+(class a {}) + 1;
+(class extends b {}) + 1;
+(class a extends b {}) + 1;