Make defs private and add nullability
Make defs private and add nullability

rachel-fenichel/blockly,mark-friedman/blockly,rachel-fenichel/blockly,rachel-fenichel/blockly,rachel-fenichel/blockly,google/blockly,google/blockly,google/blockly,google/blockly,rachel-fenichel/blockly,mark-friedman/blockly,google/blockly,mark-friedman/blockly,mark-friedman/blockly,google/blockly,rachel-fenichel/blockly,mark-friedman/blockly,google/blockly,rachel-fenichel/blockly,mark-friedman/blockly