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.

从 runtime/vm/service/service.md 中 VM service protocol 的 Library 部分开始，然后跟踪 issue 中提到的变量窗口和 VmServiceInterface 入口点。检查求值框如何处理 globals，包括 Web 平台的 lazy getters；完成的要求是就如何将 globals 与 locals 分开显示并覆盖受影响的实现达成一致。

由索引模型根据 Issue 内容生成。