fixes #173, ability to use native JS indexers

test/codegen/expect/domtest.js

+var domtest = dart.defineLibrary(domtest, {});
+var dom = dart.import(dom);
+var core = dart.import(core);
+(function(exports, dom, core) {
+  'use strict';
+  // Function testNativeIndexers: () → dynamic
+  function testNativeIndexers() {
+    let nodes = dom.document.querySelector('body').childNodes;
+    for (let i = 0; dart.notNull(i) < dart.notNull(nodes.length); i = dart.notNull(i) + 1) {
+      core.print(nodes[i]);
+      core.print(nodes[i] = null);
+    }
+  }
+  // Exports:
+  exports.testNativeIndexers = testNativeIndexers;
+})(domtest, dom, core);