Implement modular compilation

tool/input_sdk/private/ddc_runtime/classes.dart

 // 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.
 
 /// This library defines the operations that define and manipulate Dart
 /// classes.  Included in this are:
 ///   - Generics
 ///   - Class metadata
 ///   - Extension methods
 ///
@@ skipping 250 matching lines @@
   ctor.prototype = proto;
   // Use defineProperty so we don't hit a property defined on Function,
   // like `caller` and `arguments`.
   $defineProperty($clazz, $name, { value: ctor, configurable: true });
 })()''');
 
 final _extensionType = JS('', 'Symbol("extensionType")');
 
 getExtensionType(obj) => JS('', '$obj[$_extensionType]');
 
-final dartx = JS('', '{}');
+final dartx = JS('', 'dartx');
 
 getExtensionSymbol(name) => JS('', '''(() => {
   let sym = $dartx[$name];
   if (!sym) $dartx[$name] = sym = Symbol('dartx.' + $name.toString());
   return sym;
 })()''');
 
 defineExtensionNames(names) => JS('', '$names.forEach($getExtensionSymbol)');
 
 // Install properties in prototype order.  Properties / descriptors from
@@ skipping 90 matching lines @@
 })()''');
 
 /// Sets the element type of a list literal.
 list(obj, elementType) =>
     JS('', '$setType($obj, ${getGenericClass(JSArray)}($elementType))');
 
 setBaseClass(derived, base) => JS('', '''(() => {
   // Link the extension to the type it's extending as a base class.
   $derived.prototype.__proto__ = $base.prototype;
 })()''');