Adding Blockly.FieldDropdown.validateOptions_() (#1840)
Adding Blockly.FieldDropdown.validateOptions_() (#1840)

* Adding Blockly.FieldDropdown.validateOptions_()

Verifies the datastructure of dropdown option lists.
Catches errors early, such as:
  https://groups.google.com/forum/#!topic/blockly/ae9WrvEcOP0

rachel-fenichel/blockly,trinketapp/blockly,rachel-fenichel/blockly,trinketapp/blockly,mark-friedman/blockly,trinketapp/blockly,trinketapp/blockly,picklesrus/blockly,nickmain/blockly,joshlory/blockly,twodee/blockly,nickmain/blockly,picklesrus/blockly,twodee/blockly,twodee/blockly,google/blockly,mark-friedman/blockly,google/blockly,rachel-fenichel/blockly,rachel-fenichel/blockly,twodee/blockly,nickmain/blockly,nickmain/blockly,twodee/blockly,google/blockly,nickmain/blockly,joshlory/blockly,google/blockly,rachel-fenichel/blockly,google/blockly,joshlory/blockly,rachel-fenichel/blockly,google/blockly,mark-friedman/blockly,picklesrus/blockly,twodee/blockly,nickmain/blockly,mark-friedman/blockly,mark-friedman/blockly,mark-friedman/blockly,google/blockly,rachel-fenichel/blockly