fix(build): do not copy polyfills to the dist OT unless building es5

Prior to this change Stencil will copy polyfills to the `dist` output
target whether or not the user has indicated they'll be necessary.

The polyfills comprise two things: copying the polyfills themselves into
the 'loader' path, and adding code to the lazy-loader entry points which
loads those polyfills. Instead of just assuming that the user wants
this, we now gate this behavior on whether `buildEs5` is set on the
Stencil configuration.

fixes #5416
STENCIL-1288

src/compiler/output-targets/dist-lazy/generate-esm.ts

@@ -58,14 +58,25 @@ export const generateEsm = async (
         '',
       );
 
-      await copyPolyfills(config, compilerCtx, esmOutputs);
+      if (config.buildEs5) {
+        await copyPolyfills(config, compilerCtx, esmOutputs);
+      }
       await generateShortcuts(config, compilerCtx, outputTargets, output);
     }
   }
 
   return { name: 'esm', buildCtx };
 };
 
+/**
+ * Copy polyfills from `$INSTALL_DIR/internal/client/polyfills` to the lazy
+ * loader output directory where $INSTALL_DIR is the directory in which the
+ * `@stencil/core` package is installed.
+ *
+ * @param config a validated Stencil configuration
+ * @param compilerCtx the current compiler context
+ * @param outputTargets dist-lazy output targets
+ */
 const copyPolyfills = async (
   config: d.ValidatedConfig,
   compilerCtx: d.CompilerCtx,

src/compiler/output-targets/output-lazy-loader.ts

@@ -47,23 +47,31 @@ const generateLoader = async (
   const es5EntryPoint = join(es5Dir, 'loader.js');
   const es2017EntryPoint = join(es2017Dir, 'loader.js');
   const polyfillsEntryPoint = join(es2017Dir, 'polyfills/index.js');
-  const cjsEntryPoint = join(cjsDir, 'loader.cjs.js');
   const polyfillsExport = `export * from '${relative(loaderPath, polyfillsEntryPoint)}';`;
-  const indexContent = `${generatePreamble(config)}
-${es5HtmlElement}
-${polyfillsExport}
-export * from '${relative(loaderPath, es5EntryPoint)}';
-`;
-  const indexES2017Content = `${generatePreamble(config)}
-${polyfillsExport}
-export * from '${relative(loaderPath, es2017EntryPoint)}';
-`;
-  const indexCjsContent = `${generatePreamble(config)}
-module.exports = require('${relative(loaderPath, cjsEntryPoint)}');
-module.exports.applyPolyfills = function() { return Promise.resolve() };
-`;
+
+  const cjsEntryPoint = join(cjsDir, 'loader.cjs.js');
+
+  const indexContent = filterAndJoin([
+    generatePreamble(config),
+    es5HtmlElement,
+    config.buildEs5 ? polyfillsExport : null,
+    `export * from '${relative(loaderPath, es5EntryPoint)}';`,
+  ]);
+
+  const indexES2017Content = filterAndJoin([
+    generatePreamble(config),
+    config.buildEs5 ? polyfillsExport : null,
+    `export * from '${relative(loaderPath, es2017EntryPoint)}';`,
+  ]);
+
+  const indexCjsContent = filterAndJoin([
+    generatePreamble(config),
+    `module.exports = require('${relative(loaderPath, cjsEntryPoint)}');`,
+    config.buildEs5 ? `module.exports.applyPolyfills = function() { return Promise.resolve() };` : null,
+  ]);
 
   const indexDtsPath = join(loaderPath, 'index.d.ts');
+
   await Promise.all([
     compilerCtx.fs.writeFile(join(loaderPath, 'package.json'), packageJsonContent),
     compilerCtx.fs.writeFile(join(loaderPath, 'index.d.ts'), generateIndexDts(indexDtsPath, outputTarget.componentDts)),
@@ -98,3 +106,15 @@ export declare function applyPolyfills(): Promise<void>;
 export declare function setNonce(nonce: string): void;
 `;
 };
+
+/**
+ * Given an array of 'parts' which can be assembled into a string 1) filter
+ * out any parts that are `null` and 2) join the remaining strings into a single
+ * output string
+ *
+ * @param parts an array of parts to filter and join
+ * @returns the joined string
+ */
+function filterAndJoin(parts: (string | null)[]): string {
+  return parts.filter((part) => part !== null).join('\n');
+}