Don't expose filesystem of the machine that deploys dart-services

[johnpryan]

The JS console reveals filesystem details:

errors.dart:202 Uncaught Error: Assertion failed: file:///Users/brettmorgan/Documents/GitHub/dart-services/flutter-sdk/packages/flutter/lib/src/widgets/framework.dart:2577:20
_debugCurrentBuildTarget == context
is not true
    at Object.throw_ [as throw] (errors.dart:251)
    at Object.assertFailed (errors.dart:29)
    at framework.dart:2577
    at framework.BuildOwner.new.buildScope (framework.dart:2580)
    at RenderObjectToWidgetAdapter.new.attachToRenderTree (binding.dart:1111)
    at binding$.WidgetsFlutterBinding.new.attachRootWidget (binding.dart:943)
    at binding.dart:924
    at internalCallback (isolate_helper.dart:48)

See #1966 for steps to reproduce

[domesticmouse]

@srawlins Any ideas on if this is even possible?

[srawlins]

When compiling DDC on the dev's workstation, we could first cd to a temp directory with a known suffix (say _foo/), then remove that text from errors (s#file:///.*_foo/##).

[johnpryan]

Related to #1365

[parlough]

This potentially is not as relevant now as the deploy is happening on Cloud Build, but it's still potentially worth investigating.

I'm going to consolidate this issue in to #3008 alongside other exception surfacing changes. Thanks for the report!