Compile import/export syntax in @babel/runtime-related modules.

Case in point: @babel/runtime/helpers/esm/typeof.js uses ECMAScript module
syntax (import, export), but must not be compiled with transforms like
@babel/plugin-transform-typeof-symbol, since it's part of the runtime
library depended upon by that transform.

This logic is an extreme implementation detail for sure, but at least
babel-compiler is the only code that needs to know about this complexity.

packages/babel-compiler/babel-compiler.js

@@ -77,9 +77,10 @@ BCp.processOneFileForTarget = function (inputFile, source) {
       ! toBeAdded.bare &&
       // If you need to exclude a specific file within an app from Babel
       // compilation, give it the following file extension: .es5.js
-      ! isInputFilePathExcluded(inputFilePath)) {
-    var extraFeatures = Object.assign({}, this.extraFeatures);
-    var arch = inputFile.getArch();
+      ! excludedFileExtensionPattern.test(inputFilePath)) {
+
+    const extraFeatures = { ...this.extraFeatures };
+    const arch = inputFile.getArch();
 
     if (arch.startsWith("os.")) {
       // Start with a much simpler set of Babel presets and plugins if
@@ -96,6 +97,14 @@ BCp.processOneFileForTarget = function (inputFile, source) {
       extraFeatures.jscript = true;
     }
 
+    if (shouldCompileModulesOnly(inputFilePath)) {
+      // Modules like @babel/runtime/helpers/esm/typeof.js need to be
+      // compiled to support ECMAScript modules syntax, but should *not*
+      // be compiled in any other way (for more explanation, see my longer
+      // comment in shouldCompileModulesOnly).
+      extraFeatures.compileModulesOnly = true;
+    }
+
     var babelOptions = Babel.getDefaultOptions(extraFeatures);
     babelOptions.caller = { name: "meteor", arch };
 
@@ -165,11 +174,7 @@ BCp.processOneFileForTarget = function (inputFile, source) {
   return toBeAdded;
 };
 
-function isInputFilePathExcluded(path) {
-  if (excludedFileExtensionPattern.test(path)) {
-    return true;
-  }
-
+function shouldCompileModulesOnly(path) {
   const parts = path.split("/");
   const nmi = parts.lastIndexOf("node_modules");
   if (nmi >= 0) {