Initial --modules=es6 support

test/codegen/expect/es6_modules.js

+'use strict';
+const exports = {};
+import dart from "dart/_runtime";
+import core from "dart/core";
+let dartx = dart.dartx;
+const Callback = dart.typedef('Callback', () => dart.functionType(dart.void, [], {i: core.int}));
+class A extends core.Object {}
+class _A extends core.Object {}
+const B$ = dart.generic(function(T) {
+  class B extends core.Object {}
+  return B;
+});
+let B = B$();
+const _B$ = dart.generic(function(T) {
+  class _B extends core.Object {}
+  return _B;
+});
+let _B = _B$();
+function f() {
+}
+dart.fn(f);
+function _f() {
+}
+dart.fn(_f);
+const constant = "abc";
+exports.lazy = "abc";
+exports.mutable = "abc";
+// Exports:
+exports.Callback = Callback;
+exports.A = A;
+exports.B$ = B$;
+exports.B = B;
+exports.f = f;
+exports.constant = constant;
+export default exports;