[VM Service Protocol] Support lazily computing bound variables for frames

[grouma]

The VM Service protocol requires all bound variables to be computed for a frame. Each bound variable, for web debugging, potentially requires an evaluation in Chrome to compute which introduces latency. However, VS Code supports lazily displaying bound variables when a user selects a given frame in the stack. We should modify the VM Service protocol to support this lazy computation. We can either introduce a new call like boundVariablesFor(Frame) or introduce the concept of a boundVariableRef.

[grouma]

Related to dart-lang/webdev#993 and #43826

[bkonyi]

As proposed by @jacob314 offline:

A simpler way to provide this API could be to provide one version of the getStackTrace API that does not specify any bound variables. The field is already optional in Frame.

[devoncarew]

OK, I see the definition of getStack() here: https://github.com/dart-lang/sdk/blob/master/runtime/vm/service/service.md#getstack. The proposal is to add something like an optional don'tPopulateVariables parameter to the call?

How would users go from an unpopulated Frame (https://github.com/dart-lang/sdk/blob/master/runtime/vm/service/service.md#frame) then to one populated with bound variables? Would the getObject() call return a populated frame? I see that Frame is a Response, and doesn't have an object ID.

cc @jacob314 @bkonyi @grouma

[jacob314]

The simplest solution is getStack could have a starting frame index as well as an ending frame index which matches what other debugger protocols support. Adding a frameId would also be reasonable and the debugger adapter protocol supports both. Both would be easy enough for dwds to support.