Use destructuring assignments for named parameters (#180)

lib/runtime/dart/_js_helper.js

Index: lib/runtime/dart/_js_helper.js
diff --git a/lib/runtime/dart/_js_helper.js b/lib/runtime/dart/_js_helper.js
index 9c0b80121381bb158fc0386fffd91314bdd693be..55fbaf9b2695a2ca638e0daa01916efa06dedad2 100644
--- a/lib/runtime/dart/_js_helper.js
+++ b/lib/runtime/dart/_js_helper.js
@@ -31,8 +31,7 @@ dart_library.library('dart/_js_helper', null, /* Imports */[
     constructors: () => ({Native: [Native, [core.String]]})
   });
   class JsPeerInterface extends core.Object {
-    JsPeerInterface(opts) {
-      let name = opts && 'name' in opts ? opts.name : null;
+    JsPeerInterface({name = null} = {}) {
       this.name = name;
     }
   }
@@ -79,9 +78,7 @@ dart_library.library('dart/_js_helper', null, /* Imports */[
     toString() {
       return `RegExp/${this.pattern}/`;
     }
-    JSSyntaxRegExp(source, opts) {
-      let multiLine = opts && 'multiLine' in opts ? opts.multiLine : false;
-      let caseSensitive = opts && 'caseSensitive' in opts ? opts.caseSensitive : true;
+    JSSyntaxRegExp(source, {multiLine = false, caseSensitive = true} = {}) {
       this.pattern = source;
       this[_nativeRegExp] = JSSyntaxRegExp.makeNative(source, multiLine, caseSensitive, false);
       this[_nativeGlobalRegExp] = null;