Add --modules=node support

lib/runtime/dart/math.js

Index: lib/runtime/dart/math.js
diff --git a/lib/runtime/dart/math.js b/lib/runtime/dart/math.js
index 02adc431061f78b16f7f56d8fc4552289b4ab3ef..e863864c114e70d6be2c8e1de0bf45e58c294523 100644
--- a/lib/runtime/dart/math.js
+++ b/lib/runtime/dart/math.js
@@ -88,7 +88,7 @@ dart_library.library('dart/math', null, /* Imports */[
     });
     return Point;
   });
-  let Point = Point$();
+  const Point = Point$();
   class Random extends core.Object {
     static new(seed) {
       if (seed === void 0) seed = null;
@@ -173,7 +173,7 @@ dart_library.library('dart/math', null, /* Imports */[
     });
     return _RectangleBase;
   });
-  let _RectangleBase = _RectangleBase$();
+  const _RectangleBase = _RectangleBase$();
   const Rectangle$ = dart.generic(function(T) {
     class Rectangle extends _RectangleBase$(T) {
       Rectangle(left, top, width, height) {
@@ -199,7 +199,7 @@ dart_library.library('dart/math', null, /* Imports */[
     });
     return Rectangle;
   });
-  let Rectangle = Rectangle$();
+  const Rectangle = Rectangle$();
   const _width = Symbol('_width');
   const _height = Symbol('_height');
   const MutableRectangle$ = dart.generic(function(T) {
@@ -244,7 +244,7 @@ dart_library.library('dart/math', null, /* Imports */[
     });
     return MutableRectangle;
   });
-  let MutableRectangle = MutableRectangle$();
+  const MutableRectangle = MutableRectangle$();
   function _clampToZero(value) {
     dart.assert(dart.notNull(value) < 0);
     return -dart.notNull(value) * 0;