Update to latest dart 2.0 version

[pyrossh]

There are breaking changes in dart 2.0. Butterfly needs to be updated to support it. Any plans on how to do it. I'm getting these errors now,

pyros2097 hello[master !?]> webdev serve

[INFO] Setting up file watchers completed, took 15ms
[INFO] Waiting for all file watchers to be ready completed, took 9ms
[INFO] Building new asset graph completed, took 708ms
[INFO] Checking for unexpected pre-existing outputs. completed, took 1ms
[SEVERE] build_web_compilers|ddc on package:butterfly/butterfly.module:
Error compiling dartdevc module:butterfly|lib/butterfly.ddc.js

[error] The expression in an assert must be of type 'bool'. (package:butterfly/src/convenience.dart, line 336, col 12)
[error] The expression in an assert must be of type 'bool'. (package:butterfly/src/key.dart, line 93, col 12)
[error] The expression in an assert must be of type 'bool'. (package:butterfly/src/key.dart, line 105, col 12)
[error] The expression in an assert must be of type 'bool'. (package:butterfly/src/style.dart, line 84, col 14)
[error] The expression in an assert must be of type 'bool'. (package:butterfly/src/style.dart, line 120, col 14)
[error] The expression in an assert must be of type 'bool'. (package:butterfly/src/tree.dart, line 87, col 12)
[error] The expression in an assert must be of type 'bool'. (package:butterfly/src/widget.dart, line 62, col 12)

Please fix all errors before compiling (warnings are okay).
}
[WARNING] build_web_compilers|entrypoint on web/hello.dart:
Unable to read butterfly|lib/butterfly.ddc.js, check your console or the `.dart_tool/build/generated/butterfly/lib/butterfly.ddc.js.errors` log file.
[INFO] Running build completed, took 7.4s
[INFO] Caching finalized dependency graph completed, took 147ms
[INFO] Succeeded after 7.6s with 19 outputs (1263 actions)
Serving `web` on http://localhost:8080
[INFO] Terminating. No further builds will be scheduled
[INFO] Builds finished. Safe to exit