Add ES8/ES2017 target (#10768)

src/compiler/commandLineParser.ts

@@ -262,7 +262,9 @@ namespace ts {
                 "es3": ScriptTarget.ES3,
                 "es5": ScriptTarget.ES5,
                 "es6": ScriptTarget.ES6,
+                "es8": ScriptTarget.ES8,
                 "es2015": ScriptTarget.ES2015,
+                "es2017": ScriptTarget.ES2017,
             }),
             description: Diagnostics.Specify_ECMAScript_target_version_Colon_ES3_default_ES5_or_ES2015,
             paramType: Diagnostics.VERSION,