Better error message for const loop variables
Better error message for const loop variables

issue 5629

The new error message is:


'file:/n.dart': Error: line 4 pos 8: Loop variable cannot be 'const'
  for (const i in [1,2,3]) {
       ^
Review URL: https://codereview.chromium.org//11027036

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@13255 260f80e4-7a28-3924-810f-c04153c831b5

dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk