Support using scope in evaluateInFrame

[annagrin]

Support using scope in evaluateInFrame to allow DevTools eval-and-browse feature to work on web platform: flutter/devtools#4962

Previously, to implement VmService.evaluateInFrame, we used Runtime.evaluateOnCallFrame chrome remote debugging API to evaluate the dart expression compiled to JS. Unfortunately, this API does not allow passing additional scope, so we did not support passing additional scope for vm_service.evaluateInFrame on web platform. This worked well until the need for passing additional scope to expression evaluation surfaced in eval-and-browse feature in dart DevTools.

This change supports passing scope to VmService.evaluateInFrame by:

• Allow passing non-empty scope in ExpressionEvaluator.evaluateExpressionInFrame.
  • Create class JsBuilder for efficient JS code building.
  • Use JsBuilder in expression evaluator to create JS expressions to pass to chrome DevTools API.
  • If scope was passed to evaluateExpressionInFrame:
    • collect all variables currently in scope from WipCallFrame object.
    • create a JS function that takes the scope as params and call it using
      Runtime.callFunctionOn API, with scope variables passed as arguments.
    • handle this parameter specially by using JS bind().
• Keep the original implementation for empty scope case as an optimization.
• Add tests.

Details

For example, to evaluate a dart expression

this.t + a + x + y

in a dart scope that defines a and this, and additional scope x, y, we

• compile dart function

  (x, y, a) { return this.t + a + x + y; } 

  to JavaScript function

  jsFunc

  using the expression compiler (i.e. frontend server or expression compiler worker).

• create JavaScript wrapper function, jsWrapperFunc, defined as

  function (x, y, a, __t$this) {
    try {
      return function (x, y, a) {
        return jsFunc(x, y, a);
      }.bind(__t$this)(x, y, a);
    } catch (error) {
      return error.name + ": " + error.message;
    }
  }

• collect scope variable object IDs for total scope (frame scope from WipCallFrame object + additional scope)

• call jsWrapperFunc using Runtime.callFunctionOn chrome API with scope variable object IDs passed as arguments.

Closes: #1344
Towards: flutter/devtools#4962

[elliette]

LGTM with a couple comments

[jacob314]

Looks good overall. Looking forward to this support!