JS: Format if statements with no else on a single line

test/codegen/expect/methods.js

 dart_library.library('methods', null, /* Imports */[
   "dart/_runtime",
   'dart/core'
 ], /* Lazy imports */[
 ], function(exports, dart, core) {
   'use strict';
   let dartx = dart.dartx;
   const _c = Symbol('_c');
   class A extends core.Object {
     A() {
       this[_c] = 3;
     }
     x() {
       return 42;
     }
     y(a) {
       return a;
     }
     z(b) {
-      if (b === void 0)
-        b = null;
+      if (b === void 0) b = null;
       return dart.asInt(b);
     }
     zz(b) {
-      if (b === void 0)
-        b = 0;
+      if (b === void 0) b = 0;
       return b;
     }
     w(a, opts) {
       let b = opts && 'b' in opts ? opts.b : null;
       return dart.asInt(dart.notNull(a) + dart.notNull(b));
     }
     clashWithObjectProperty(opts) {
       let constructor = opts && 'constructor' in opts ? opts.constructor : null;
       return constructor;
     }
@@ skipping 48 matching lines @@
     let c = dart.bind("", dartx.padLeft);
     let r = dart.bind(3.0, dartx.floor);
   }
   dart.fn(test);
   // Exports:
   exports.A = A;
   exports.Bar = Bar;
   exports.Foo = Foo;
   exports.test = test;
 });