title: Find error when error message doesn't show location
body: <p>I'm building an app using Ionic, Angular, and Firebase.</p>
  
  <p>I'm having a problem where the code is being built and packaged by webpack and it returns an error from <code>polyfills.js</code>, which is not useful in the slightest...</p>
  
  <p>The error message looks like this: </p>
  
  <pre><code>Unhandled Promise rejection: Missing Command Error ; Zone: &lt;root&gt; ; Task:
  Promise.then ; Value: Missing Command Error undefined
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/TQcrL.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/TQcrL.png" alt="enter image description here"></a></p>
  
  <p>The error happens every 30ish seconds. </p>
  
  <p>And the non-minifed version of the error looks like this:</p>
  
  <pre><code>function l(t) {
      var e = t.prototype,
          n = e.then;
      e[d] = n;
      var r = Object.getOwnPropertyDescriptor(t.prototype, "then");
      r &amp;&amp; !1 === r.writable &amp;&amp; r.configurable &amp;&amp; Object.defineProperty(t.prototype, "then", {
              writable: !0
          }),
          t.prototype.then = function(t, e) {
              var r = this;
              return new x(function(t, e) {
                  n.call(r, t, e)
              }).then(t, e)
          },
          t[R] = !0
  }
  var h = n.symbol,
      p = [],
      v = h("Promise"),
      d = h("then");
  n.onUnhandledError = function(t) {
          if (n.showUncaughtError()) {
              var e = t &amp;&amp; t.rejection;
              e ? console.error("Unhandled Promise rejection:", e instanceof Error ? e.message : e, "; Zone:", t.zone.name, "; Task:", t.task &amp;&amp; t.task.source, "; Value:", e, e instanceof Error ? e.stack : void 0) : console.error(t)
          }
      },
      n.microtaskDrainDone = function() {
          for (; p.length;)
              for (; p.length;)
                  ! function() {
                      var t = p.shift();
                      try {
                          t.zone.runGuarded(function() {
                              throw t
                          })
                      } catch (t) {
                          r(t)
                      }
                  }()
      };
  </code></pre>
  
  <p>Any idea how I would debug this? Thank you!</p>