add test cases for es2017.string

Author: imcotton

tests/cases/conformance/es2017/useStringPadding1.ts

@@ -0,0 +1,7 @@
+// @target: es5
+// @lib: es5,es2017.string
+
+var str = 'foo';
+
+str.padStart(5, '-');
+str.padEnd(5, '-');

tests/cases/conformance/es2017/useStringPadding2.ts

@@ -0,0 +1,7 @@
+// @target: es5
+// @lib: es5
+
+var str = 'foo';
+
+str.padStart(5, '-');
+str.padEnd(5, '-');

tests/cases/conformance/es2017/useStringPadding3.ts

@@ -0,0 +1,7 @@
+// @target: es6
+//
+
+var str = 'foo';
+
+str.padStart(5, '-');
+str.padEnd(5, '-');

tests/cases/conformance/es2017/useStringPadding4.ts

@@ -0,0 +1,7 @@
+// @target: es6
+// @lib: es2017
+
+var str = 'foo';
+
+str.padStart(5, '-');
+str.padEnd(5, '-');