Fixed Proxy when field/method starts with _

If trying to access a JavaScript field/variable that starts with _ (underscore), Dart treats it a library private and member name it suffixed with '@internalLibraryIdentifier' which we have to strip before sending over to the JS side.
Merge pull request #54 from pavelgj/master

Fixed Proxy when field/variable starts with _
bump pubspec
add test for private members
Merge pull request #55 from dart-lang/private_test

add test for private members
replace String.concat with operator +
Merge pull request #56 from a14n/fix-deprecations

replace String.concat with operator +
Merge branch 'master' into wrapping
fix deprecations and dart changes