Consult babel-runtime package when discarding helper modules.

The most fool-proof way to tell if a module is provided by babel-runtime
is to load babel-runtime as an isopacket and ask it.

This should fix any remaining issues like this one:
https://github.com/meteor/meteor/pull/7668#pullrequestreview-4379559

tools/isobuild/compiler-plugin.js

@@ -771,6 +771,15 @@ class ResourceSlot {
   }
 }
 
+let babelRuntime;
+function checkBabelRuntimeHelper(id) {
+  if (! babelRuntime) {
+    babelRuntime = require("../tool-env/isopackets.js")
+      .load("runtime")["babel-runtime"];
+  }
+  return babelRuntime.checkHelper(id);
+}
+
 export class PackageSourceBatch {
   constructor(unibuild, processor, {
     sourceRoot,
@@ -1112,10 +1121,7 @@ export class PackageSourceBatch {
             // relying on programs/server/npm/node_modules/babel-runtime,
             // but on the web these bundled files are all we have, so we'd
             // better not remove them.
-            if (parts[0] === "node_modules" &&
-                parts[1] === "babel-runtime" &&
-                (parts[2] === "helpers" ||
-                 parts[2].startsWith("regenerator"))) {
+            if (checkBabelRuntimeHelper(file.installPath)) {
               return;
             }
           }

tools/tool-env/isopackets.js

@@ -56,7 +56,8 @@ export const ISOPACKETS = {
   'constraint-solver': ['constraint-solver'],
   'cordova-support': ['boilerplate-generator', 'logging', 'webapp-hashing',
                       'xmlbuilder'],
-  'logging': ['logging']
+  'logging': ['logging'],
+  'runtime': ['babel-runtime'],
 };
 
 // Caches isopackets in memory (each isopacket only needs to be loaded