task model: override inference not working for final fields when subtype narrows the type

[jmesserly]

  testChecker(
      'infer from RHS only if it wont conflict with overridden fields 2',
      {
        '/main.dart': '''
        class A {
          final x;
        }

        class B extends A {
          final x = 2;
        }

        foo() {
          String y = /*severe:StaticTypeError*/new B().x;
          int z = new B().x;
        }
    '''
      },
      inferFromOverrides: true);

Naturally non-final fields need to be invariant. But subtypes should be allowed to narrow final fields, right? The test above expects B.x to be inferred as int because that doesn't violate the contract of A.x

[jmesserly]

https://codereview.chromium.org/1367183004 is the fix. It seems harmless to infer here but a bit ad hoc