Support handling classDecl fromJson(String s) in JsonDecodable

[morlay]

To support usage like

@JsonCodable()
class Account {
    AccountType accountType
}

class AccountType {
  static const USER = AccountType("USER");
  static const AGENT = AccountType("AGENT");

  const AccountType(this.value);

  final String value;

  factory AccountType.fromJson(String value) {
    return AccountType(value);
  }

  String toJson() {
    return value;
  }
}

[copybara-service]

Thank you for your contribution! This project uses Gerrit for code reviews. Your pull request has automatically been converted into a code review at:

https://dart-review.googlesource.com/c/sdk/+/383930

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Additional commits pushed to this PR will update both the PR and the corresponding Gerrit CL. After the review is complete on the CL, your reviewer will merge the CL (automatically closing this PR).

[copybara-service]

Thank you for your contribution! This project uses Gerrit for code reviews. Your pull request has automatically been converted into a code review at:

https://dart-review.googlesource.com/c/sdk/+/384161

Please wait for a developer to review your code review at the above link; you can speed up the review if you sign into Gerrit and manually add a reviewer that has recently worked on the relevant code. See CONTRIBUTING.md to learn how to upload changes to Gerrit directly.

Additional commits pushed to this PR will update both the PR and the corresponding Gerrit CL. After the review is complete on the CL, your reviewer will merge the CL (automatically closing this PR).

[jakemac53]

Reviewed at the Gerrit CL, https://dart-review.googlesource.com/c/sdk/+/384161

[mit-mit]

Hi @morlay are you still working on this? If so, please respond in the Gerrit link.

[morlay]

Hi @morlay are you still working on this? If so, please respond in the Gerrit link.

Sorry. I have network issue to access googlesource.
Could you help to take over this PR?

[copybara-service]

https://dart-review.googlesource.com/c/sdk/+/384161 has been updated with the latest commits from this pull request.

[copybara-service]

https://dart-review.googlesource.com/c/sdk/+/384161 has been updated with the latest commits from this pull request.