Use destructuring assignments for named parameters (#180)

lib/runtime/dart/mirrors.js

 dart_library.library('dart/mirrors', null, /* Imports */[
   "dart/_runtime",
   'dart/core'
 ], /* Lazy imports */[
   'dart/_js_mirrors'
 ], function(exports, dart, core, _js_mirrors) {
   'use strict';
   let dartx = dart.dartx;
   class MirrorSystem extends core.Object {
     findLibrary(libraryName) {
@@ skipping 79 matching lines @@
     Comment(text, trimmedText, isDocComment) {
       this.text = text;
       this.trimmedText = trimmedText;
       this.isDocComment = isDocComment;
     }
   }
   dart.setSignature(Comment, {
     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;
       this.override = override;
     }
   }
   dart.setSignature(MirrorsUsed, {
     constructors: () => ({MirrorsUsed: [MirrorsUsed, [], {symbols: dart.dynamic, targets: dart.dynamic, metaTargets: dart.dynamic, override: dart.dynamic}]})
   });
   // Exports:
@@ skipping 16 matching lines @@
   exports.FunctionTypeMirror = FunctionTypeMirror;
   exports.TypeVariableMirror = TypeVariableMirror;
   exports.TypedefMirror = TypedefMirror;
   exports.MethodMirror = MethodMirror;
   exports.VariableMirror = VariableMirror;
   exports.ParameterMirror = ParameterMirror;
   exports.SourceLocation = SourceLocation;
   exports.Comment = Comment;
   exports.MirrorsUsed = MirrorsUsed;
 });