Report a compile-time error if a const field is multiply initialized.

Report a compile-time error if a const field is multiply initialized.

This CL makes analyzer consistent with the VM and the spec, by
producing a compile-time error for code such as:

    class C {
      final x = 1;
      const C() : x = 2;
    }
    main() {
      const C();
    }

Note that Dart2js also produces an error in this circumstance, but its
error is too general (the error should only be produced if the const
constructor is invoked using "const"; if the const constructor is
invoked using "new", or is not invoked at all, it should only be a
warning).  See #23618.

R=brianwilkerson@google.com

Committed: https://github.com/dart-lang/sdk/commit/2f610a3ff53413efa688b8e58e0842f599e6c8fa

[Paul Berry, 2015-06-10 20:50:19]

paulberry@google.com changed reviewers:
+ brianwilkerson@google.com

[Brian Wilkerson, 2015-06-10 20:55:57]

LGTM

[Paul Berry, 2015-06-10 21:02:48]

Committed patchset #1 (id:1) manually as
2f610a3ff53413efa688b8e58e0842f599e6c8fa (presubmit successful).