Use destructuring assignments for named parameters (#180)

lib/runtime/dart/mirrors.js

Index: lib/runtime/dart/mirrors.js
diff --git a/lib/runtime/dart/mirrors.js b/lib/runtime/dart/mirrors.js
index 42ac300e35c984709e3ad21b8afe43589b2fe8d0..584fa58450046cc6a1e5a79d89baab07d18e3c50 100644
--- a/lib/runtime/dart/mirrors.js
+++ b/lib/runtime/dart/mirrors.js
@@ -97,11 +97,7 @@ dart_library.library('dart/mirrors', null, /* Imports */[
     constructors: () => ({Comment: [Comment, [core.String, core.String, core.bool]]})
   });
   class MirrorsUsed extends core.Object {
-    MirrorsUsed(opts) {
-      let symbols = opts && 'symbols' in opts ? opts.symbols : null;
-      let targets = opts && 'targets' in opts ? opts.targets : null;
-      let metaTargets = opts && 'metaTargets' in opts ? opts.metaTargets : null;
-      let override = opts && 'override' in opts ? opts.override : null;
+    MirrorsUsed({symbols = null, targets = null, metaTargets = null, override = null} = {}) {
       this.symbols = symbols;
       this.targets = targets;
       this.metaTargets = metaTargets;