Add generic type parameter to startsWith/endsWith

Author: LinusU

src/lib/es2015.core.d.ts

@@ -401,6 +401,7 @@ interface String {
      * same as the corresponding elements of this object (converted to a String) starting at
      * endPosition – length(this). Otherwise returns false.
      */
+    endsWith<T extends string>(searchString: T): this is `${string}${T}`;
     endsWith(searchString: string, endPosition?: number): boolean;
 
     /**
@@ -431,6 +432,7 @@ interface String {
      * same as the corresponding elements of this object (converted to a String) starting at
      * position. Otherwise returns false.
      */
+    startsWith<T extends string>(searchString: T, position?: 0): this is `${T}${string}`;
     startsWith(searchString: string, position?: number): boolean;
 
     /**

tests/baselines/reference/taggedTemplateStringsWithOverloadResolution3_ES6.errors.txt

@@ -113,7 +113,7 @@ tests/cases/conformance/es6/templates/taggedTemplateStringsWithOverloadResolutio
     fn5 `${ (n) => n.toFixed() }`; // will error; 'n' should have type 'string'.
                      ~~~~~~~
 !!! error TS2551: Property 'toFixed' does not exist on type 'string'. Did you mean 'fixed'?
-!!! related TS2728 /.ts/lib.es2015.core.d.ts:485:5: 'fixed' is declared here.
+!!! related TS2728 /.ts/lib.es2015.core.d.ts:487:5: 'fixed' is declared here.
     fn5 `${ (n) => n.substr(0) }`;