Mystery Frog error: We are sorry, but... can not be property

[rakudrama]

It took me quite a while to figure this out (in a larger program):

bug2.dart:
class Foo {
  Bar bar;
  Foo() {
    Bar = new Bar();
  }
}

main() {
  var x = new Foo();
}

class Bar {
}

sra@limpopo:~/play1$ ../dart-all/dart/frog/frogsh --out=t.js bug2.dart
bug2.dart:12:1: We are sorry, but... can not be property
class Bar {
^^^^^^^^^^^^

node.js:134
        throw e; // process.nextTick error, or 'error' event on first tick
        ^
CompilerException: bug2.dart:12:1: We are sorry, but... can not be property
class Bar {
^^^^^^^^^^^^
    at World._lang_message (/usr/local/google/home/sra/dart-all/dart/frog/frogsh:15707:5)
    at World.internalError (/usr/local/google/home/sra/dart-all/dart/frog/frogsh:15726:8)
    at TypeMember.providePropertySyntax (/usr/local/google/home/sra/dart-all/dart/frog/frogsh:8958:16)
    at MemberSet.get$treatAsField (/usr/local/google/home/sra/dart-all/dart/frog/frogsh:9948:16)
    at MethodGenerator._visitVarAssign (/usr/local/google/home/sra/dart-all/dart/frog/frogsh:8129:18)
    at MethodGenerator._visitAssign (/usr/local/google/home/sra/dart-all/dart/frog/frogsh:8101:17)
    at MethodGenerator.visitBinaryExpression (/usr/local/google/home/sra/dart-all/dart/frog/frogsh:8090:17)
    at BinaryExpression.visit (/usr/local/google/home/sra/dart-all/dart/frog/frogsh:13837:18)
    at MethodGenerator.visitValue (/usr/local/google/home/sra/dart-all/dart/frog/frogsh:7566:20)
    at MethodGenerator.visitVoid (/usr/local/google/home/sra/dart-all/dart/frog/frogsh:7581:15)

The problem is that the assignment at line 4 has the class name as a LHS. 'Bar' should be 'bar'.
The diagnostic should indicate the line with the error. If 'Bar' has been underlined I think I would have got it.

Maybe line 12 could be a secondary diagnostic, e.g.

bug2.dart:4:4: class 'Bar' can not be target of assignment
    Bar = new Bar();
    ^^^
bug2.dart:12:1: class 'Bar' defined here.

The VM does a reasonable job:

'bug2.dart': Error: line 4 pos 9: Left hand side of '=' is not assignable
    Bar = new Bar();
        ^

[DartBot]

This comment was originally written by drfibonacci@google.com

---

Added Triaged label.

[jmesserly]

Yeah, that was actually a bogus internal error obscuring the better error we would've displayed later. Fix coming soon.

[jmesserly]

Set owner to @jmesserly.

[jmesserly]

Added Fixed label.