have web.dart export the helpers; deprecate helpers.dart

[devoncarew]

• have web.dart export the generated code (now in src/dom.dart) and the helpers (now in src/helpers.dart)
• deprecate helpers.dart
• close Do not export web.dart from helpers.dart #98

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[devoncarew]

This leaves us with:

• web.dart
• helpers.dart (deprecated)
• src/dom.dart
• src/helpers.dart

[srujzs]

Looks good, thanks again!