Added valueOf definitions for String and Number

src/lib/core.d.ts

@@ -410,6 +410,9 @@ interface String {
       */
     substr(from: number, length?: number): string;
 
+    /** Returns the primitive value of the specified object. */
+    valueOf(): string;
+
     [index: number]: string;
 }
 
@@ -462,6 +465,9 @@ interface Number {
       * @param precision Number of significant digits. Must be in the range 1 - 21, inclusive.
       */
     toPrecision(precision?: number): string;
+
+    /** Returns the primitive value of the specified object. */
+    valueOf(): number;
 }
 
 interface NumberConstructor {