Add ES8/ES2017 target (microsoft#10768)

Author: andrejbaran

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,

src/compiler/core.ts

@@ -1191,7 +1191,7 @@ namespace ts {
     export function getEmitModuleKind(compilerOptions: CompilerOptions) {
         return typeof compilerOptions.module === "number" ?
             compilerOptions.module :
-            getEmitScriptTarget(compilerOptions) === ScriptTarget.ES6 ? ModuleKind.ES6 : ModuleKind.CommonJS;
+            getEmitScriptTarget(compilerOptions) >= ScriptTarget.ES6 ? ModuleKind.ES6 : ModuleKind.CommonJS;
     }
 
     /* @internal */

src/compiler/emitter.ts

@@ -2192,7 +2192,10 @@ const _super = (function (geti, seti) {
                 helpersEmitted = true;
             }
 
-            if (!awaiterEmitted && node.flags & NodeFlags.HasAsyncFunctions) {
+            // Only emit __awaiter function when target ES5/ES6.
+            // Only emit __generator function when target ES5.
+            // For target ES8 and above, we can emit async/await as is.
+            if (!awaiterEmitted && node.flags & NodeFlags.HasAsyncFunctions && (languageVersion < ScriptTarget.ES8)) {
                 writeLines(awaiterHelper);
                 if (languageVersion < ScriptTarget.ES6) {
                     writeLines(generatorHelper);

src/compiler/transformer.ts

@@ -115,7 +115,9 @@ namespace ts {
             transformers.push(transformJsx);
         }
 
-        transformers.push(transformES7);
+        if (languageVersion < ScriptTarget.ES8) {
+            transformers.push(transformES7);
+        }
 
         if (languageVersion < ScriptTarget.ES6) {
             transformers.push(transformES6);
@@ -339,4 +341,4 @@ namespace ts {
             return statements;
         }
     }
-}
+}

src/compiler/transformers/ts.ts

@@ -240,11 +240,14 @@ namespace ts {
                     // ES6 export and default modifiers are elided when inside a namespace.
                     return currentNamespace ? undefined : node;
 
+                case SyntaxKind.AsyncKeyword:
+                    // ES8 async keyword
+                    return languageVersion < ScriptTarget.ES8 ? undefined : node;
+
                 case SyntaxKind.PublicKeyword:
                 case SyntaxKind.PrivateKeyword:
                 case SyntaxKind.ProtectedKeyword:
                 case SyntaxKind.AbstractKeyword:
-                case SyntaxKind.AsyncKeyword:
                 case SyntaxKind.ConstKeyword:
                 case SyntaxKind.DeclareKeyword:
                 case SyntaxKind.ReadonlyKeyword:
@@ -2235,7 +2238,7 @@ namespace ts {
          */
         function visitArrowFunction(node: ArrowFunction) {
             const func = createArrowFunction(
-                /*modifiers*/ undefined,
+                visitNodes(node.modifiers, visitor, isModifier),
                 /*typeParameters*/ undefined,
                 visitNodes(node.parameters, visitor, isParameter),
                 /*type*/ undefined,
@@ -2250,7 +2253,7 @@ namespace ts {
         }
 
         function transformFunctionBody(node: MethodDeclaration | AccessorDeclaration | FunctionDeclaration | FunctionExpression): FunctionBody {
-            if (isAsyncFunctionLike(node)) {
+            if (isAsyncFunctionLike(node) && languageVersion < ScriptTarget.ES8) {
                 return <FunctionBody>transformAsyncFunctionBody(node);
             }
 
@@ -2270,7 +2273,7 @@ namespace ts {
         }
 
         function transformConciseBody(node: ArrowFunction): ConciseBody {
-            if (isAsyncFunctionLike(node)) {
+            if (isAsyncFunctionLike(node) && languageVersion < ScriptTarget.ES8) {
                 return transformAsyncFunctionBody(node);
             }
 
@@ -2453,14 +2456,22 @@ namespace ts {
          * @param node The await expression node.
          */
         function visitAwaitExpression(node: AwaitExpression): Expression {
-            return setOriginalNode(
-                createYield(
-                    /*asteriskToken*/ undefined,
-                    visitNode(node.expression, visitor, isExpression),
-                    /*location*/ node
-                ),
-                node
-            );
+            return languageVersion >= ScriptTarget.ES8
+                ? setOriginalNode(
+                    createAwait(
+                        visitNode(node.expression, visitor, isExpression),
+                        /*location*/ node
+                    ),
+                    node
+                )
+                : setOriginalNode(
+                    createYield(
+                        /*asteriskToken*/ undefined,
+                        visitNode(node.expression, visitor, isExpression),
+                        /*location*/ node
+                    ),
+                    node
+                );
         }
 
         /**

src/compiler/types.ts

@@ -2831,8 +2831,10 @@ namespace ts {
         ES3 = 0,
         ES5 = 1,
         ES6 = 2,
+        ES8 = 3,
         ES2015 = ES6,
-        Latest = ES6,
+        ES2017 = ES8,
+        Latest = ES8,
     }
 
     export const enum LanguageVariant {