replace boot.js for boot.dart

[DartBot]

<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#18007

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this is blocked by a couple issues:

 - issue dart-lang/sdk#18001: we need to be able to use spawnDomUri with package imports
 - issue dart-lang/sdk#18006: we want to include boot.dart in polymer.html in the same way we include boot.js

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2049220?v=3" align="left" width="48" height="48"hspace="10"> Comment by sigmundch

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Seems that some our plans might be changing here. I deleted the portions of boot.dart that were equivalent to boot.js in the change that reintroduces initPOlymer.

In case it is useful in the future, here is what the code looked like when I deleted it:

    main() {
      var scripts = discoverScripts(document, window.location.href);
      var sb = new StringBuffer()..write('library bootstrap;\n\n');
      int count = 0;
      for (var s in scripts) {
        sb.writeln("import '${s.importUrl}' as prefix_$count;");
        count++;
      }
      sb.writeln("import 'package:polymer/src/mirror_loader.dart';");
      sb.write('\nmain() => startPolymerInDevelopment([\n');
      for (var s in scripts) {
        sb.writeln(" '${s.importUrl}',");
      }
      sb.write(']);\n');
      var isolateUri = _asDataUri(sb.toString());
      spawnDomUri(Uri.parse(isolateUri), [], '');
    }

[DartBot]

<img src="https://avatars.githubusercontent.com/u/1081711?v=3" align="left" width="48" height="48"hspace="10"> Comment by jmesserly

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Removed Area-Polymer label.
Added Pkg-Polymer, Area-Pkg labels.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2049220?v=3" align="left" width="48" height="48"hspace="10"> Comment by sigmundch

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Closing this until we set our new plan with the new script semantics.

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Added NotPlanned label.