fix(d.ts): enable angular2 compilation with TS flag --strictNullChecks (#8902)

Eliminate the following compiler error when using TS 1.9+ with strict null checking enabled:

node_modules/@angular/core/src/util/decorators.d.ts(9,5): error TS2411: Property 'extends' of type 'Type | undefined' is not assignable to string index type 'Type | Function | any[]'.

https://github.com/angular/angular/issues/8720

modules/@angular/core/src/util/decorators.ts

@@ -26,7 +26,7 @@ export interface ClassDefinition {
    * Other methods on the class. Note that values should have type 'Function' but TS requires
    * all properties to have a narrower type than the index signature.
    */
-  [x: string]: Type | Function | any[];
+  [x: string]: Type | Function | any[] | undefined;
 }
 
 /**