Remove eval() use in BlockEditor

[chynu]

The updatePreview() function under block_editor_controller.js uses eval() to evaluate manually inputted block definitions in order to display the user-defined blocks in the preview. Remove the use of eval() in parsing and displaying user-inputted block definitions.

[AnmAtAnm]

Potential solution would be to use JSInterpreter. This would require wrapping a whole lot of functions to support the majority of Blockly blocks. It would also take a significant test infrastructure to verify.

[AnmAtAnm]

See temporary solution in google/blockly#1235.