Compile error in latest flutter Getter not found: 'JSON'.

[ryanhz]

=== BUILD TARGET Runner OF PROJECT Runner WITH CONFIGURATION Debug ===
compiler message: file:///Users/Ryan/Applications/flutter/.pub-cache/hosted/pub.dartlang.org/slack-1.2.2/lib/src/slacksrc.dart:51:12: Error: Getter not found: 'JSON'.
compiler message:     return JSON.encode(message);
compiler message:            ^^^^
compiler message: file:///Users/Ryan/Applications/flutter/.pub-cache/hosted/pub.dartlang.org/slack-1.2.2/lib/src/slacksrc.dart:51:12: Error: The getter 'JSON' isn't defined for the class '#lib1::Message'.
compiler message: Try correcting the name to the name of an existing getter, or defining a getter or field named 'JSON'.
compiler message:     return JSON.encode(message);
compiler message:            ^
compiler message: file:///Users/Ryan/Applications/flutter/.pub-cache/hosted/pub.dartlang.org/slack-1.2.2/lib/src/slacksrc.dart:87:24: Error: Getter not found: 'JSON'.
compiler message:   String toString() => JSON.encode(_toMap());
compiler message:                        ^^^^
compiler message: file:///Users/Ryan/Applications/flutter/.pub-cache/hosted/pub.dartlang.org/slack-1.2.2/lib/src/slacksrc.dart:87:24: Error: The getter 'JSON' isn't defined for the class '#lib1::Attachment'.
compiler message: Try correcting the name to the name of an existing getter, or defining a getter or field named 'JSON'.
compiler message:   String toString() => JSON.encode(_toMap());
compiler message:                        ^
compiler message: file:///Users/Ryan/Applications/flutter/.pub-cache/hosted/pub.dartlang.org/slack-1.2.2/lib/src/slacksrc.dart:125:24: Error: Getter not found: 'JSON'.
compiler message:   String toString() => JSON.encode(_toMap());
compiler message:                        ^^^^
compiler message: file:///Users/Ryan/Applications/flutter/.pub-cache/hosted/pub.dartlang.org/slack-1.2.2/lib/src/slacksrc.dart:125:24: Error: The getter 'JSON' isn't defined for the class '#lib1::Field'.
compiler message: Try correcting the name to the name of an existing getter, or defining a getter or field named 'JSON'.
compiler message:   String toString() => JSON.encode(_toMap());