Minor reorganization of vendor dependencies.

frontend/vendor/main.externs.js

@@ -0,0 +1,52 @@
+var TopLevel = {
+  "offsetX": function() {},
+  "offsetY": function() {},
+  "nativeEvent": function() {},
+  "dataTransfer": function() {},
+  "dropEffect": function() {},
+  "offsetHeight": function() {},
+  "offsetWidth": function() {},
+  "scrollTop": function() {},
+  "scrollLeft": function() {},
+  "refs": function() {},
+  "ctrlKey": function() {},
+  "shiftKey": function() {},
+  "keyCode": function() {},
+  "currentTarget": function() {},
+  "createObjectURL": function() {},
+  "URL": function() {},
+  "innerHTML": function() {},
+  "getElementById": function() {},
+  "clientX": function() {},
+  "clientY": function() {},
+  "href": function() {},
+  "identifier": function() {},
+  "getBoundingClientRect": function() {},
+  "getBBox": function() {},
+  "addEventListener": function() {},
+  "postMessage": function() {},
+  "data": function() {},
+  "querySelector": function() {},
+  "outerHTML": function() {},
+  "tagName": function() {},
+  "appendChild": function() {},
+  "createElement": function() {},
+  "createElementNS": function() {},
+  "viewBox": function() {},
+  "baseVal": function() {},
+  "x": function() {},
+  "y": function() {},
+  "width": function() {},
+  "height": function() {},
+  "type": function() {},
+  "stopPropagation": function() {},
+  "preventDefault": function() {},
+  "innerText": function() {},
+  "target": function() {},
+  "value": function() {},
+  "click": function() {},
+  "result": function() {},
+  "readAsText": function() {},
+  "src": function() {},
+  "readAsDataURL": function() {}
+};