Remove binding for nonexistent SyntheticDataTransfer class

[greglittlefield-wf]

Problem

There's no SyntheticDataTransfer object in React JS; synthetic JS mouse events get passed the actual DataTransfer object.

Having this JS binding and typing the real DataTransfer object as it was causing dart2js errors that looked like:

Warning: 'gJ2' is used reflectively but not in MirrorsUsed. This will break minified code.
14:38:58.512 react_with_react_dom_prod.js:100 Uncaught Error: NoSuchMethodError: method not found: 'gJ2'

Solution

Remove the private JS binding, and use the native DataTransfer instead.

Testing

Trigger a react-dart onClick handler and verify it doesn't throw in checked mode.

• pub serve example/test/
• In both Dartium (checked mode) and Chrome (dart2js)
  • Go to http://localhost:8080/ref_test.html
  • Click a button
  • Verify that the button did something and that there are no errors in the console

[aaronlademann-wf]

+1

[robbecker-wf]

+10 Verified no errors in Chrome or dartium (dart 1)