Define "dart.library.mirrors" and "dart.library.isolate" as false in dart2js

[munificent]

Since Dart 2.0 does not support those libraries on the web (#30538), the fromEnvironment() flags for those should return false.

[jmesserly]

FYI, some dependency cleanup:
https://dart-review.googlesource.com/c/sdk/+/43581 removes dart:io dependency on dart:isolate
https://dart-review.googlesource.com/c/sdk/+/43860 removes dart:html dependency on dart:isolate

that may help if dart2js wants to disable "dart:isolate" import statically.

[dgrove]

@sigmundch is this done with the new FE?

[sigmundch]

Not yet - in fact, dart.libraries.io incorrectly returns true now that we are using the CFE

[sigmundch]

Work is in progress here. The main challenge is that we want to allow imports to these libraries and throw runtime errors instead of a static errors for a while longer. To support that we have to:

• extend CFE to answer consistently on config-specific imports based on the libraries-specification file (libraries.json)
• integrate dart2js and CFE to pass along a libraries.json file
• update how dart2js implements dart.library.* const expressions

[sigmundch]

This is now working with some ad-hoc logic based on the old *.platform library configuration files. We need to later do some cleanup to make it solely depend on the new libraries.json specification file.