Suggestion: Show global variables in variable window

Currently the vm service protocol exposes globals per library in https://github.com/dart-lang/sdk/blob/master/runtime/vm/service/service.md#library

We have an occasional ask from customers for exposing global variables for inspection in dart DevTools. Would it be possible to add them to the variable window (and separate globals and locals into different groups)?

VM service implementations currently support evaluation of globals (that are available in the current scope) in the evaluation box, so it seems feasible to show globals upfront.

Note: might need additional support from implementations of VmServiceInterface. On web platform, for example, globals are lazily in created in JavaScript and will require calling a getter. In this case we could show a `...` next to a global name and call the getter on click.

Comienza con la sección Library del VM service protocol en runtime/vm/service/service.md y, después, sigue los puntos de entrada de la ventana de variables y VmServiceInterface mencionados en el issue. Comprueba cómo el cuadro de evaluación gestiona los globals, incluidos los lazy getters de la plataforma web; el trabajo estará terminado cuando exista un enfoque acordado para mostrar los globals por separado de los locals y cubrir las implementaciones afectadas.

Escrito por el modelo de indexación a partir del texto del issue.