use js_ast instead of strings to generate JS

lib/runtime/dart_runtime.js

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 var dart;
 (function (dart) {
   var defineProperty = Object.defineProperty;
   var getOwnPropertyDescriptor = Object.getOwnPropertyDescriptor;
   var getOwnPropertyNames = Object.getOwnPropertyNames;
 
@@ skipping 20 matching lines @@
   }
 
   function dload(obj, field) {
     if (!(field in obj)) {
       throw new core.NoSuchMethodError(obj, field);
     }
     return obj[field];
   }
   dart.dload = dload;
 
+  // TODO(jmesserly): this should call noSuchMethod, not throw.
   function throwNoSuchMethod(obj, name, args, opt_func) {
     if (obj === void 0) obj = opt_func;
     throw new core.NoSuchMethodError(obj, name, args);
   }
 
   function checkAndCall(f, obj, args, name) {
     if (!(f instanceof Function)) {
       // Grab the `call` method if it's not a function.
       if (f !== null) f = f.call;
       if (!(f instanceof Function)) {
@@ skipping 208 matching lines @@
     }
     return map;
   }
 
   function assert(condition) {
     // TODO(jmesserly): throw assertion error.
     if (!condition) throw 'assertion failed';
   }
   dart.assert = assert;
 
+  function throw_(obj) { throw obj; }
+  dart.throw_ = throw_;
+
   /**
    * Given a class and an initializer method name, creates a constructor
    * function with the same name. For example `new SomeClass.name(args)`.
    */
   function defineNamedConstructor(clazz, name) {
     var proto = clazz.prototype;
     var initMethod = proto[clazz.name + '$' + name];
     var ctor = function() { return initMethod.apply(this, arguments); }
     ctor.prototype = proto;
     clazz[name] = ctor;
@@ skipping 55 matching lines @@
    * we use the same trick as named constructors, and do them as instance
    * methods that perform initialization.
    */
   // TODO(jmesserly): we'll need to rethink this once the ES6 spec and V8
   // settles. See <https://github.com/dart-lang/dart-dev-compiler/issues/51>.
   // Performance of this pattern is likely to be bad.
   dart.Object = function Object() {
     // Get the class name for this instance.
     var name = this.constructor.name;
     // Call the default constructor.
-    var result = this[name].apply(this, arguments);
+    var init = this[name];
+    var result = void 0;
+    if (init) result = init.apply(this, arguments);
     return result === void 0 ? this : result;
   };
   // The initializer for dart.Object
   dart.Object.prototype.Object = function() {};
   dart.Object.prototype.constructor = dart.Object;
 
 })(dart || (dart = {}));