Dart 2 compilation doesn't allow for graceful error message with bad `main`

Things like `int main;` or missing `main` used to have nice error messages but now hit somewhat more cryptic compilation errors.

`main` isn't a method:

before: `Top-level hybridMain is not a function.`
now: `Error: A value of type 'dart.core::int' can't be assigned to a variable of type 'dart.core::Function'.`

Missing `main`:

before: `No top-level hybridMain() function defined.`
now: `Error: Getter not found: 'main'.`

https://github.com/dart-lang/test/blob/1e41eeaeab796fe0559997abc944782db4126133/lib/src/runner/hybrid_listener.dart#L45-L53

The non-method one could be fixed by changing the [bootstrap methods](https://github.com/dart-lang/test/blob/1e41eeaeab796fe0559997abc944782db4126133/lib/src/bootstrap/node.dart#L13) from taking a `Function getMain()` to `dynamic getMain()` which lets the type issue surface in the same place it used to. The missing main would be tougher to fix.

Start with lib/src/runner/hybrid_listener.dart around the main-function checks and lib/src/bootstrap/node.dart around the bootstrap methods. Reproduce the reported cases with a non-method or missing main, then compare the current compiler errors with the earlier graceful messages. Done means both invalid-main cases produce clear, intentional errors again.

Written by the indexing model from the issue text.