Reduce (debug-only) VerifyMaps from O(n^2) to O(n).
Reduce (debug-only) VerifyMaps from O(n^2) to O(n).

VerifyMaps in dart2js identity reload 1400-1900ms -> 80-100ms.

BUG=http://dartbug.com/26807
R=johnmccutchan@google.com

Review URL: https://codereview.chromium.org/2115713002 .

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