module.exports should default to an empty object.

Per the spec in modules.md the exports property should default to an
empty object. We lazy allocate it so that modules that just replace it
don't create the empty object and then throw it away.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/872043003

engine/src/flutter/tests/modules/import-without-export.sky

@@ -3,7 +3,9 @@
 <import src="../resources/dump-as-text.sky" />
 <div id="result">FAIL - Script did not execute</div>
 <script>
-document.getElementById("result").textContent =
-    hello === undefined ? "PASS" : "FAIL";
+var result = "FAIL";
+if (typeof hello === "object" && Object.keys(hello).length === 0)
+  result = "PASS";
+document.getElementById("result").textContent = result;
 </script>
 </html>

engine/src/flutter/tests/modules/modules-expected.txt

@@ -1,5 +1,6 @@
-Running 1 tests
+Running 2 tests
 ok 1 Module should be constructable
-1 tests
-1 pass
+ok 2 Module should have an empty object by default for exports
+2 tests
+2 pass
 0 fail

engine/src/flutter/tests/modules/modules.sky

@@ -13,6 +13,13 @@ describe('Module', function() {
     assert.equal(module.document, doc2);
     assert.equal(module.url, "http://www.example.com/module");
   });
+
+  it('should have an empty object by default for exports', function() {
+    var app = new Application(new Document(), "http://www.example.com/app");
+    var module = new Module(app, new Document(), "http://www.example.com/module");
+    assert.isObject(module.exports);
+    assert.lengthOf(Object.keys(module.exports), 0);
+  });
 });
 </script>
 </html>