Lower bound of generic parameter is not respected

[jacehensley-wf]

Dart Version: 1.21.0

Problem

• Lower bound of generic parameters is not respected in strong mode.

Dartpad

• https://dartpad.dartlang.org/db5a2b40d72fe968587495a5c1b39b28
• More simplified example: https://dartpad.dartlang.org/6b0a4a418e55ef7f89fab7f6e059d963

Expected Behavior

• Dart should realize that ReturnType is T.

[jacehensley-wf]

Hmm, after working out more simplified example with @greglittlefield-wf, I see that the error is valid.