Add --target esnext

Currently, this disables the rest and spread transforms. This will
change as proposals enter and leave stage 3.

Author: sandersn

src/compiler/commandLineParser.ts

@@ -265,6 +265,7 @@ namespace ts {
                 "es2015": ScriptTarget.ES2015,
                 "es2016": ScriptTarget.ES2016,
                 "es2017": ScriptTarget.ES2017,
+                "esnext": ScriptTarget.ESNext,
             }),
             description: Diagnostics.Specify_ECMAScript_target_version_Colon_ES3_default_ES5_or_ES2015,
             paramType: Diagnostics.VERSION,

src/compiler/emitter.ts

@@ -732,7 +732,7 @@ const _super = (function (geti, seti) {
                     return emitPropertyAssignment(<PropertyAssignment>node);
                 case SyntaxKind.ShorthandPropertyAssignment:
                     return emitShorthandPropertyAssignment(<ShorthandPropertyAssignment>node);
-                case SyntaxKind.ShorthandPropertyAssignment:
+                case SyntaxKind.SpreadAssignment:
                     return emitSpreadAssignment(node as SpreadAssignment);
 
                 // Enum
@@ -2214,7 +2214,10 @@ const _super = (function (geti, seti) {
                 helpersEmitted = true;
             }
 
-            if (compilerOptions.jsx !== JsxEmit.Preserve && !assignEmitted && (node.flags & NodeFlags.HasSpreadAttribute)) {
+            if ((languageVersion < ScriptTarget.ESNext || currentSourceFile.scriptKind === ScriptKind.JSX || currentSourceFile.scriptKind === ScriptKind.TSX) &&
+                compilerOptions.jsx !== JsxEmit.Preserve &&
+                !assignEmitted &&
+                node.flags & NodeFlags.HasSpreadAttribute) {
                 writeLines(assignHelper);
                 assignEmitted = true;
             }

src/compiler/transformer.ts

@@ -117,7 +117,10 @@ namespace ts {
             transformers.push(transformJsx);
         }
 
-        transformers.push(transformESNext);
+        if (languageVersion < ScriptTarget.ESNext) {
+            transformers.push(transformESNext);
+        }
+
         if (languageVersion < ScriptTarget.ES2017) {
             transformers.push(transformES2017);
         }

src/compiler/types.ts

@@ -3171,7 +3171,8 @@ namespace ts {
         ES2015 = 2,
         ES2016 = 3,
         ES2017 = 4,
-        Latest = ES2017,
+        ESNext = 5,
+        Latest = ESNext,
     }
 
     export const enum LanguageVariant {