Use destructuring assignments for named parameters (#180)

test/codegen/expect/collection/wrappers.js

Index: test/codegen/expect/collection/wrappers.js
diff --git a/test/codegen/expect/collection/wrappers.js b/test/codegen/expect/collection/wrappers.js
index d709bc10bbc6570f71a72e9c3311d0b8068d56b9..fce81fbce26a199f004a5f552a938c4aa0500fe8 100644
--- a/test/codegen/expect/collection/wrappers.js
+++ b/test/codegen/expect/collection/wrappers.js
@@ -37,9 +37,8 @@ dart_library.library('collection/wrappers', null, /* Imports */[
       get first() {
         return this[_base][dartx.first];
       }
-      firstWhere(test, opts) {
+      firstWhere(test, {orElse = null} = {}) {
         dart.as(test, dart.functionType(core.bool, [E]));
-        let orElse = opts && 'orElse' in opts ? opts.orElse : null;
         dart.as(orElse, dart.functionType(E, []));
         return this[_base][dartx.firstWhere](test, {orElse: orElse});
       }
@@ -71,9 +70,8 @@ dart_library.library('collection/wrappers', null, /* Imports */[
       get last() {
         return this[_base][dartx.last];
       }
-      lastWhere(test, opts) {
+      lastWhere(test, {orElse = null} = {}) {
         dart.as(test, dart.functionType(core.bool, [E]));
-        let orElse = opts && 'orElse' in opts ? opts.orElse : null;
         dart.as(orElse, dart.functionType(E, []));
         return this[_base][dartx.lastWhere](test, {orElse: orElse});
       }
@@ -109,8 +107,7 @@ dart_library.library('collection/wrappers', null, /* Imports */[
         dart.as(test, dart.functionType(core.bool, [E]));
         return this[_base][dartx.takeWhile](test);
       }
-      toList(opts) {
-        let growable = opts && 'growable' in opts ? opts.growable : true;
+      toList({growable = true} = {}) {
         return this[_base][dartx.toList]({growable: growable});
       }
       toSet() {
@@ -700,7 +697,7 @@ dart_library.library('collection/wrappers', null, /* Imports */[
         this[_baseMap].putIfAbsent(key, dart.as(dart.fn(() => {
           result = true;
           return value;
-        }), __CastType0));
+        }), __CastType0$(V)));
         return result;
       }
       addAll(elements) {