I think this is a dart2js issue but not 100% sure. Consider the following program:

import 'dart:mirrors';

main() {
  for (var field in reflect(new FooBar()).type.variables.values) {
    print('field ${field.simpleName} has metadata '
        '${field.metadata.map((m) => m.reflectee).join()}');
  }
}

const hello = 'hello';

class FooBar {
  @­hello var x, y, z;
}

On VM it prints:

field Symbol("y") has metadata hello
field Symbol("x") has metadata hello
field Symbol("z") has metadata hello

On dart2js+d8 it prints:

field Symbol("x") has metadata hello
field Symbol("y") has metadata
field Symbol("z") has metadata

Note that "y" and "z" are missing metadata.

Since you can only specify metadata on the declaredIdentifier in the variableDeclaration (http://www.dartlang.org/docs/spec/latest/dart-language-specification.html#h.55kzc4r0q21p) , it makes me think metadata should apply to all of the identifiers.