Add includes method to String interface, and remove contains

src/lib/es6.d.ts

@@ -377,7 +377,7 @@ interface String {
       * @param searchString search string 
       * @param position If position is undefined, 0 is assumed, so as to search all of the String.
       */
-    contains(searchString: string, position?: number): boolean;
+    includes(searchString: string, position?: number): boolean;
 
     /**
       * Returns true if the sequence of elements of searchString converted to a String is the 

tests/baselines/reference/stringIncludes.js

@@ -0,0 +1,10 @@
+//// [stringIncludes.ts]
+
+var includes: boolean;
+includes = "abcde".includes("cd");
+includes = "abcde".includes("cd", 2);
+
+//// [stringIncludes.js]
+var includes;
+includes = "abcde".includes("cd");
+includes = "abcde".includes("cd", 2);

tests/baselines/reference/stringIncludes.symbols

@@ -0,0 +1,15 @@
+=== tests/cases/compiler/stringIncludes.ts ===
+
+var includes: boolean;
+>includes : Symbol(includes, Decl(stringIncludes.ts, 1, 3))
+
+includes = "abcde".includes("cd");
+>includes : Symbol(includes, Decl(stringIncludes.ts, 1, 3))
+>"abcde".includes : Symbol(String.includes, Decl(lib.d.ts, 1569, 37))
+>includes : Symbol(String.includes, Decl(lib.d.ts, 1569, 37))
+
+includes = "abcde".includes("cd", 2);
+>includes : Symbol(includes, Decl(stringIncludes.ts, 1, 3))
+>"abcde".includes : Symbol(String.includes, Decl(lib.d.ts, 1569, 37))
+>includes : Symbol(String.includes, Decl(lib.d.ts, 1569, 37))
+

tests/baselines/reference/stringIncludes.types

@@ -0,0 +1,24 @@
+=== tests/cases/compiler/stringIncludes.ts ===
+
+var includes: boolean;
+>includes : boolean
+
+includes = "abcde".includes("cd");
+>includes = "abcde".includes("cd") : boolean
+>includes : boolean
+>"abcde".includes("cd") : boolean
+>"abcde".includes : (searchString: string, position?: number) => boolean
+>"abcde" : string
+>includes : (searchString: string, position?: number) => boolean
+>"cd" : string
+
+includes = "abcde".includes("cd", 2);
+>includes = "abcde".includes("cd", 2) : boolean
+>includes : boolean
+>"abcde".includes("cd", 2) : boolean
+>"abcde".includes : (searchString: string, position?: number) => boolean
+>"abcde" : string
+>includes : (searchString: string, position?: number) => boolean
+>"cd" : string
+>2 : number
+

tests/cases/compiler/stringIncludes.ts

@@ -0,0 +1,5 @@
+//@target: ES6
+
+var includes: boolean;
+includes = "abcde".includes("cd");
+includes = "abcde".includes("cd", 2);