test "should compile for random heap modifications" failing on Node v6

[Manu Sridharan]

After I apply the change in #2, I still see a test failure under Node v6, for test "should compile for random heap modifications." Here is the stack:

TypeError: Cannot convert a Symbol value to a string
      at Field.toString (/Users/msridhar/git-repos/mimic/bin/Data.js:167:55)
      at Assign.toString (/Users/msridhar/git-repos/mimic/bin/Data.js:888:73)
      at Array.join (native)
      at Seq.toString (/Users/msridhar/git-repos/mimic/bin/Data.js:775:27)
      at Program.toString (/Users/msridhar/git-repos/mimic/bin/Data.js:1173:38)
      at Object.compile (/Users/msridhar/git-repos/mimic/bin/Compile.js:8:26)
      at Context.<anonymous> (/Users/msridhar/git-repos/mimic/bin/test/Test.js:150:26)
      at callFn (/Users/msridhar/git-repos/mimic/node_modules/mocha/lib/runnable.js:250:21)