[dev_compiler] Support widget inspector via expression compilation

[annagrin]

Issue

Widget inspector for web platform does not show layout

Repro

• Create a flutter project with following code:

// @dart=2.9

import 'package:flutter/widgets.dart';

void main() {
  return runApp(
    Column(children: [
    const Center(
      child:
        Text('Hello, world!',
          key: Key('title'),
          textDirection: TextDirection.ltr
        )
      )
    ]
    ));
}

• Start debugging in VSCode (for web plaftorm)
• Open devtools widget inspector page
• Observe empty layout

Expected result (from iPhone simulator):

How to see the evaluated expression and result in chrome

• Start VSCode on the app code above
• Set the verbose mode in launch.json flutter configuration:

{
  "name": "Flutter",
  "request": "launch",
   "type": "dart",
    "args": ["-v"],
}

• Set a breakpoint inside initServiceExtensions in
  flutter/flutter/packages/flutter/lib/src/widgets/widget_inspector.dart
• Set flutter to run in chrome (web-javascript)
• Start debugging
• On breakpoint, copy and paste the dart expression (starting after return library. and ending before next line) from the information in the following bug:

flutter/devtools#2627

• Copy and paste the JS expression from the VCCode debug console window to the chrome console window
• Observe result:
  VM973:50 Uncaught SyntaxError: Identifier 'e' has already been declared

[annagrin]

Work needed for this:

• SDK: (ExpressionCompiler, FrontendServer, ExpressionCompilerWorker) support expression compilation in library scope (no line)
• SDK (ddc) make sure we generate JS expression that evaluates correctly and matches VM functionality
• dwds: use expression compiler in the implementation of the VM Service evaluate() API
• flutter tools: update dwds

[jacob314]

You should be able to get past the e has already been declared error by renaming the (e) in the various catch blocks to each have unique names in the expression being evaluated.

[franklinyow]

@sigmundch Can you assign an owner?