Move from `pub build` to `dart2js` for Observatory

[natebosch]

If there are any Transformers they will need to be rewritten as Builders, then build_runner and related packages will need to be pulled in to the SDK.

There are no Transformers so we should be able to call Dart2Js directly.

[dgrove]

@devoncarew @a-siva can you assign someone?

[natebosch]

From the pubspec there aren't any necessary Transformers, the HTML already references main.dart.js to the dart_to_js_script_rewriter Transformer isn't doing anything today.

We should be able to rewrite the build to invoke dart2js directly. Unfortunately I don't know what the process is that normally builds this.

[a-siva]

/cc @rmacnak-google

[natebosch]

I've made some progress on this but I'm getting stuck.

I have been making changes to observatory_tool.py to find the dart2js snapshot in the same way as it does for pub and then invoke that for the "build" action.

I'm able to get to the point where I run dart2js but I'm hitting an error that makes me think maybe the dart2js snapshot was somehow build incorrectly.

Command failed: /usr/local/google/home/nbosch/projects/dart-sdk/sdk/tools/sdks/linux/dart-sdk/bin/dart /usr/local/google/home/nbosch/projects/dart-sdk/sdk/tools/sdks/linux/dart-sdk/bin/snapshots/dart2js.dart.snapshot -DOBS_VER=2.0.0-edge -o /usr/local/google/home/nbosch/projects/dart-sdk/sdk/out/ReleaseX64/observatory/build/web/main.dart.js /usr/local/google/home/nbosch/projects/dart-sdk/sdk/out/ReleaseX64/gen/observatory_copy/web/main.dart
output: The compiler crashed: Error reading '../../../../../../tools/sdks/sdk/lib/dart_client.platform'  (No such file or directory)

[rmacnak-google]

Don't invoke dart2js' snapshot directly. Use the wrapper script at tools/sdks//dart-sdk/bin/dart2js, which sets extra arguments to tell dart2js where to find the core libraries etc.