Nested functions are not immutable

[floitschG]

dart2js does not catch that nested functions are immutable.

The following program "works":
===
main() {
  foo() {
    foo = () => 499;
    print(foo());
  }
  foo();
}
===

For more complicated programs the compiler crashes.

[sigmundch]

Just sent out a possible fix in https://codereview.chromium.org/1073983003/, but I'm not sure if it is the approach you want to use here.

I'm basically marking the element of named local functions as final, so no setter is available.

[sigmundch]

fixed in r45123

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Added Fixed label.