make pub-build robust to accept transformer code with bugs or errors

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<img src="https://avatars.githubusercontent.com/u/2049220?v=3" align="left" width="96" height="96"hspace="10"> Issue by sigmundch
Originally opened as dart-lang/sdk#14206

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I saw this error as a result of issue #674:

Because pub tries to load 'polymer.dart', the vm gives this error and pub-build fails immediately:

IsolateSpawnException: 'The built-in library 'dart:html' is not available on the stand-alone VM.
'http://127.0.0.1:45866/packages/polymer/polymer.dart': Error: line 44 pos 1: library handler failed
import 'dart:html';
^
'file:///tmp/pub_DuxgFe/runInIsolate.dart': Error: line 1 pos 1: library handler failed
import "http://127.0.0.1:45866/packages/polymer/polymer.dart&quot;;import 'dart:async';
^'

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<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="48" height="48"hspace="10"> Comment by nex3

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Making pub behave in a way that's any better than crash here would require infrastructure for reloading transformers after the initialization, which isn't planned for the near future.

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Removed this from the M8 milestone.
Added this to the Later milestone.

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<img src="https://avatars.githubusercontent.com/u/46275?v=3" align="left" width="48" height="48"hspace="10"> Comment by munificent

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Removed Priority-Unassigned label.
Added Priority-Medium label.

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<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="48" height="48"hspace="10"> Comment by nex3

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Marked this as being blocked by dart-lang/sdk#19163.

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<img src="https://avatars.githubusercontent.com/u/2156198?v=3" align="left" width="48" height="48"hspace="10"> Comment by kasperl

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Removed this from the Later milestone.
Added Oldschool-Milestone-Later label.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2156198?v=3" align="left" width="48" height="48"hspace="10"> Comment by kasperl

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Removed Oldschool-Milestone-Later label.

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<img src="https://avatars.githubusercontent.com/u/46275?v=3" align="left" width="48" height="48"hspace="10"> Comment by munificent

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I'm going to close this because we don't have any plans to change this behavior any time soon. If we do eventually support reloading transformers, this will get fixed implicitly by that.

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Added NotPlanned label.