Revert "Remove binding for nonexistent SyntheticDataTransfer class"

[evanweible-wf]

Reverts #151 - it results in some typing errors like so:

Unhandled exception:
type 'JSObjectImpl' is not a subtype of type 'DataTransfer' of 'dt' where
JSObjectImpl is from dart:js
DataTransfer is from dart:html
#0      syntheticDataTransferFactory (package:react/react_client.dart:746:65)
#1      syntheticMouseEventFactory (package:react/react_client.dart:784:30)
#2      _convertEventHandlers.<anonymous closure>.<anonymous closure>.<anonymous closure> (package:react/react_client.dart:561:35)

This commit had not yet been released, so for the time being (in order to get #153 through) we are going to revert. We can circle back to fix this by finding a way to construct a real DataTransfer object using JS.

@greglittlefield-wf @aaronlademann-wf @kealjones-wk @corwinsheahan-wf

[greglittlefield-wf]

+1