Compile-time error when using assignment initializer with anonymous functions.

[hcbpassos]

Seems like Dart analysis gets confused when using assignment initializer with anonymous functions. For instance:

class Foo {
  final VoidCallback bar;
  Foo() : bar = () {}; // Expected an identifier.
}

When wrapping the function in parenthesis, the error goes away:

class Foo {
  final VoidCallback bar;
  Foo() : bar = (() {}); // Fine.
}

I'm not sure whether this behavior is expected, but I cannot see any reason why it should be.

[lrhn]

The problem here is that the parser has a hard time distinguishing function bodies in the initializer list from a constructor body.

It's not impossible, but it's hard enough that Dart has so far not allowed unparenthesized function-expressions in initializer lists.

The example here is easier because there is no argument, but

 Foo(x) : bar = (x) { print(x); };

requires arbitrary look-ahead to see whether the {print(x)} is a function body or a constructor body. (It's probably a function body because of the trailing ;).

I'm not sure the grammar reflects this restriction. @eernstg?

[eernstg]

This restriction has been part of Dart since 2013 (cf. dart-lang/sdk#11509), but the language specification does not mention it, and never did. This PR finally adds it.