Make ExpressionCompiler parameter in Dwds.start required

Currently, if it is null, Dwds will revert to previous behavior of throwing unimplemented exception on expression evaluation. After expression all scenarios in flutter support both Dwds and frontend_server, make this parameter required to avoid future breaks.

See:

Not yet - I will change it to be required after a version of dwds including those changes is picked up by flutter/flutter, to simplify merging. If expressionCompiler is not given, dwds will simply fall back to previous behavior on evaluteInFrame (ie throwing unsupported exception)

_Originally posted by @annagrin in https://github.com/dart-lang/webdev/pull/879_

Start at the Dwds.start entry point and inspect its expressionCompiler parameter and callers, including evaluateInFrame behavior mentioned in the issue. The change is complete when expressionCompiler is required and the existing fallback behavior is no longer possible, while expression evaluation continues to work for the supported scenarios.

Written by the indexing model from the issue text.