Add --modules=node support

lib/runtime/dart/core.js

Index: lib/runtime/dart/core.js
diff --git a/lib/runtime/dart/core.js b/lib/runtime/dart/core.js
index 3b22ddac2e3152436e92b2491c51aacfda9b08f2..91f446b32197b0cf87dfa9f6dc4420993db4b654 100644
--- a/lib/runtime/dart/core.js
+++ b/lib/runtime/dart/core.js
@@ -87,7 +87,7 @@ dart_library.library('dart/core', null, /* Imports */[
     const Comparator = dart.typedef('Comparator', () => dart.functionType(int, [T, T]));
     return Comparator;
   });
-  let Comparator = Comparator$();
+  const Comparator = Comparator$();
   const Comparable$ = dart.generic(function(T) {
     class Comparable extends Object {
       static compare(a, b) {
@@ -100,7 +100,7 @@ dart_library.library('dart/core', null, /* Imports */[
     });
     return Comparable;
   });
-  let Comparable = Comparable$();
+  const Comparable = Comparable$();
   class DateTime extends Object {
     DateTime(year, month, day, hour, minute, second, millisecond) {
       if (month === void 0) month = 1;
@@ -1126,7 +1126,7 @@ dart_library.library('dart/core', null, /* Imports */[
     });
     return Expando;
   });
-  let Expando = Expando$();
+  const Expando = Expando$();
   Expando._KEY_PROPERTY_NAME = 'expando$key';
   Expando._EXPANDO_PROPERTY_NAME = 'expando$values';
   Expando._keyCount = 0;
@@ -1210,12 +1210,12 @@ dart_library.library('dart/core', null, /* Imports */[
     });
     return Iterable;
   });
-  let Iterable = Iterable$();
+  const Iterable = Iterable$();
   const _Generator$ = dart.generic(function(E) {
     const _Generator = dart.typedef('_Generator', () => dart.functionType(E, [int]));
     return _Generator;
   });
-  let _Generator = _Generator$();
+  const _Generator = _Generator$();
   const _end = dart.JsSymbol('_end');
   const _start = dart.JsSymbol('_start');
   const _generator = dart.JsSymbol('_generator');
@@ -1306,18 +1306,18 @@ dart_library.library('dart/core', null, /* Imports */[
     });
     return _GeneratorIterator;
   });
-  let _GeneratorIterator = _GeneratorIterator$();
+  const _GeneratorIterator = _GeneratorIterator$();
   const BidirectionalIterator$ = dart.generic(function(E) {
     class BidirectionalIterator extends Object {}
     BidirectionalIterator[dart.implements] = () => [Iterator$(E)];
     return BidirectionalIterator;
   });
-  let BidirectionalIterator = BidirectionalIterator$();
+  const BidirectionalIterator = BidirectionalIterator$();
   const Iterator$ = dart.generic(function(E) {
     class Iterator extends Object {}
     return Iterator;
   });
-  let Iterator = Iterator$();
+  const Iterator = Iterator$();
   const List$ = dart.generic(function(E) {
     class List extends Object {
       static new(length) {
@@ -1380,7 +1380,7 @@ dart_library.library('dart/core', null, /* Imports */[
     });
     return List;
   });
-  let List = List$();
+  const List = List$();
   const Map$ = dart.generic(function(K, V) {
     class Map extends Object {
       static new() {
@@ -1410,7 +1410,7 @@ dart_library.library('dart/core', null, /* Imports */[
     });
     return Map;
   });
-  let Map = Map$();
+  const Map = Map$();
   class Null extends Object {
     static _uninstantiable() {
       dart.throw(new UnsupportedError('class Null cannot be instantiated'));
@@ -1472,7 +1472,7 @@ dart_library.library('dart/core', null, /* Imports */[
     class Sink extends Object {}
     return Sink;
   });
-  let Sink = Sink$();
+  const Sink = Sink$();
   class StackTrace extends Object {}
   const _stop = dart.JsSymbol('_stop');
   class Stopwatch extends Object {