Add assertion for variable index of reused potentially reused AST nodes.
Add assertion for variable index of reused potentially reused AST nodes.

The flow graph inliner reuses the callee ASTs, therefore nodes may already
have an index assigned. Make sure it matches.

R=hausner@google.com

Review URL: https://codereview.chromium.org//1065303002

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@44995 260f80e4-7a28-3924-810f-c04153c831b5

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