fix(typings): Minor issues preventing angular2.d.ts from working in TS 1.4.

This removes some, but not all, of the manual work needed to patch up our
.d.ts for pushing to DefinitelyTyped. Remaining manual steps are:
- some types still missing
- declaration of decorators
- remove destructuring args

See #2686.

docs/dgeni-package/services/tsParser/getExportDocType.js

@@ -45,7 +45,9 @@ module.exports = function getExportDocType(log) {
     var node = symbol.valueDeclaration;
     while(node) {
       if ( node.flags & 0x2000 /* const */) {
-        return 'const';
+        // DefinitelyTyped is still TS 1.4 so const is not allowed.
+        // https://github.com/borisyankov/DefinitelyTyped/issues/4564
+        return 'var'; // change to const when targetting TS 1.5
       }
       node = node.parent;
     }