fix #626, add AMD module format and make it default

test/codegen_expected/js_test.js

Index: test/codegen_expected/js_test.js
diff --git a/test/codegen_expected/js_test.js b/test/codegen_expected/js_test.js
index 0c6a6854ae66ffc3bb4d224622bd6d0fb857cb23..99a9db44be6a4bacb0909bda1e13f4f805ea8756 100644
--- a/test/codegen_expected/js_test.js
+++ b/test/codegen_expected/js_test.js
@@ -1 +1,11 @@
-//FAILED TO COMPILE
+define([], function() {
+  'use strict';
+  const js_test = Object.create(null);
+  js_test.main = function() {
+    throw Error("DDC Compilation Error: js_test has 328 errors");

[vsm, 2016/08/25 12:19:24]

Should this have been generated?  I think this code is gone from
codegen_test.dart now.

[Jennifer Messerly, 2016/08/25 16:21:39]

good catch. we don't delete codegen_expect when something fails, unlike
gen/codegen_output. I'll revert.

+  };
+  // Exports:
+  return {
+    js_test: js_test
+  };
+});