Add babel for let/const lowering

This reverts commit 8623e3ee6936bf15418fc437b45d7621ca732296.

Author: jakebailey

Gulpfile.js

@@ -162,14 +162,17 @@ function getCopyrightHeader() {
  * @param {string} entrypoint
  * @param {string} outfile
  * @param {boolean} exportIsTsObject True if this file exports the TS object and should have relevant code injected.
+ * @param {boolean} performanceMatters True if this is a bundle where performance matters, so should be optimized at the cost of build time.
  */
-function esbuildTask(entrypoint, outfile, exportIsTsObject = false) {
+function esbuildTask(entrypoint, outfile, exportIsTsObject = false, performanceMatters = false) {
+    const preBabel = `${outfile}.tmp.js`;
+
     /** @type {esbuild.BuildOptions} */
     const options = {
         entryPoints: [entrypoint],
         banner: { js: getCopyrightHeader() },
         bundle: true,
-        outfile,
+        outfile: performanceMatters ? preBabel : outfile,
         platform: "node",
         // TODO: also specify minimal browser targets
         target: "node10", // Node 10 is the oldest benchmarker.
@@ -206,7 +209,13 @@ function esbuildTask(entrypoint, outfile, exportIsTsObject = false) {
 
     // TODO: these need to have better function names, for gulp.
     return {
-        build: () => esbuild.build(options),
+        build: async () => {
+            await esbuild.build(options);
+            if (performanceMatters) {
+                await exec(process.execPath, ["./node_modules/@babel/cli/bin/babel.js", preBabel, "--out-file", outfile, "--plugins", "@babel/plugin-transform-block-scoping", "--compact", "false", "--source-maps"]);
+                await del([preBabel, `${preBabel}.map`]);
+            }
+        },
         clean: () => del([outfile, `${outfile}.map`]),
         watch: () => esbuild.build({ ...options, watch: true }),
     };
@@ -238,7 +247,7 @@ cleanTasks.push(cleanDebugTools);
 const lkgPreBuild = parallel(generateLibs, series(buildScripts, generateDiagnostics, buildDebugTools));
 
 
-const esbuildTsc = esbuildTask("./src/tsc/tsc.ts", "./built/local/tsc.js", /* exportIsTsObject */ true);
+const esbuildTsc = esbuildTask("./src/tsc/tsc.ts", "./built/local/tsc.js", /* exportIsTsObject */ true, /* performanceMatters */ true);
 
 
 const buildTsc = () => cmdLineOptions.bundle ? esbuildTsc.build() : buildProject("src/tsc");
@@ -260,7 +269,7 @@ const localPreBuild = parallel(generateLibs, series(buildScripts, generateDiagno
 // Pre-build steps to use based on supplied options.
 const preBuild = cmdLineOptions.lkg ? lkgPreBuild : localPreBuild;
 
-const esbuildServices = esbuildTask("./src/typescript/typescript.ts", "./built/local/typescript.js", /* exportIsTsObject */ true);
+const esbuildServices = esbuildTask("./src/typescript/typescript.ts", "./built/local/typescript.js", /* exportIsTsObject */ true, /* performanceMatters */ true);
 
 const buildServices = () => cmdLineOptions.bundle ? esbuildServices.build() : buildProject("src/typescript");
 
@@ -285,7 +294,7 @@ task("watch-services").flags = {
 };
 
 
-const esbuildServer = esbuildTask("./src/tsserver/server.ts", "./built/local/tsserver.js", /* exportIsTsObject */ true);
+const esbuildServer = esbuildTask("./src/tsserver/server.ts", "./built/local/tsserver.js", /* exportIsTsObject */ true, /* performanceMatters */ true);
 
 const buildServer = () => cmdLineOptions.bundle ? esbuildServer.build() : buildProject("src/tsserver");
 buildServer.displayName = "buildServer";
@@ -323,7 +332,7 @@ task("watch-min").flags = {
     "   --built": "Compile using the built version of the compiler."
 };
 
-const esbuildLssl = esbuildTask("./src/tsserverlibrary/tsserverlibrary.ts", "./built/local/tsserverlibrary.js", /* exportIsTsObject */ true);
+const esbuildLssl = esbuildTask("./src/tsserverlibrary/tsserverlibrary.ts", "./built/local/tsserverlibrary.js", /* exportIsTsObject */ true, /* performanceMatters */ true);
 
 const buildLssl = () => cmdLineOptions.bundle ? esbuildLssl.build() : buildProject("src/tsserverlibrary");
 task("lssl", series(preBuild, buildLssl));