Call requestGarbageCollection in Isopack#_writeTool.

This method appears to be causing large spikes in memory consumption on
Circle CI during the `meteor --get-ready` preparation step, which often
leads to the test process being killed.

Also added a call in IsopackCache#_loadLocalPackage for good measure.

We're now calling requestGarbageCollection pretty frequently when
we run Node with --expose-gc, but that currently only happens during
Circle CI tests, so I don't think we need to implement the improvements
suggested in tools/utils/gc.js, yet.

Previously: 35f488e140, f6df21ff1e

tools/isobuild/isopack-cache.js

@@ -383,6 +383,8 @@ export class IsopackCache {
               });
             }
           }
+
+          requestGarbageCollection();
         }
       }
 

tools/isobuild/isopack.js

@@ -14,6 +14,7 @@ var utils = require('../utils/utils.js');
 var buildPluginModule = require('./build-plugin.js');
 var Console = require('../console/console.js').Console;
 var Profile = require('../tool-env/profile.js').Profile;
+import { requestGarbageCollection } from "../utils/gc.js";
 
 var rejectBadPath = function (p) {
   if (p.match(/\.\./)) {
@@ -1802,6 +1803,8 @@ _.extend(Isopack.prototype, {
       symlink: false
     });
 
+    requestGarbageCollection();
+
     // Build all of the isopackets now, so that no build step is required when
     // you're actually running meteor from a release in order to load packages.
     var isopacketBuildContext = isopackets.makeIsopacketBuildContext();
@@ -1812,6 +1815,8 @@ _.extend(Isopack.prototype, {
       // process are going to be the current tool's isopackets, not the
       // isopackets that we're writing out.
       _.each(isopackets.ISOPACKETS, function (packages, isopacketName) {
+        requestGarbageCollection();
+
         buildmessage.enterJob({
           title: "compiling " + isopacketName + " packages for the tool"
         }, function () {
@@ -1830,6 +1835,8 @@ _.extend(Isopack.prototype, {
             return;
           }
 
+          requestGarbageCollection();
+
           image.write(
             builder.enter(files.pathJoin('isopackets', isopacketName)));
         });