fix(typings): repair broken typechecks

We had the typechecker disabled by accident, and many problems snuck in

Fixes #4507

Closes #4508

modules/angular2/src/core/facade/lang.ts

@@ -58,7 +58,7 @@ export function CONST_EXPR<T>(expr: T): T {
   return expr;
 }
 
-export function CONST(): ClassDecorator {
+export function CONST(): ClassDecorator & PropertyDecorator {
   return (target) => target;
 }
 

modules/angular2/src/core/util/decorators.ts

@@ -18,7 +18,13 @@ export interface ClassDefinition {
    *
    * See {@link Class} for example of usage.
    */
-  constructor: (Function | any[]);
+  constructor: Function | any[];
+
+  /**
+   * 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[];
 }
 
 /**