simplify constructors, fixes #564

test/codegen/expect/cascade.js

 dart_library.library('cascade', null, /* Imports */[
   'dart_sdk'
 ], function(exports, dart_sdk) {
   'use strict';
   const core = dart_sdk.core;
   const dart = dart_sdk.dart;
   const dartx = dart_sdk.dartx;
   const cascade = Object.create(null);
   cascade.A = class A extends core.Object {
-    A() {
+    new() {
       this.x = null;
     }
   };
   cascade.test_closure_with_mutate = function() {
     let a = new cascade.A();
     a.x = dart.fn(() => {
       core.print("hi");
       a = null;
     });
     let _ = a;
@@ skipping 61 matching lines @@
     let a = new cascade.A();
     let y = ((() => {
       a.x = dart.dsend(a.x, '+', 1);
       a.x = dart.dsend(a.x, '-', 1);
       return a;
     })());
   };
   dart.fn(cascade.test_increment, dart.void, []);
   cascade.Base$ = dart.generic(T => {
     class Base extends core.Object {
-      Base() {
+      new() {
         this.x = dart.list([], T);
       }
     }
     return Base;
   });
   cascade.Base = cascade.Base$();
   cascade.Foo = class Foo extends cascade.Base$(core.int) {
-    Foo() {
-      super.Base();
+    new() {
+      super.new();
     }
     test_final_field_generic(t) {
       this.x[dartx.add](1);
       this.x[dartx.add](2);
       this.x[dartx.add](3);
       this.x[dartx.add](4);
     }
   };
   dart.setSignature(cascade.Foo, {
     methods: () => ({test_final_field_generic: [dart.void, [dart.dynamic]]})
   });
   // Exports:
   exports.cascade = cascade;
 });