Module names and class names can conflict in generated JS

[vsmenon]

This comes up in DeltaBlue and other benchmarks:

var BenchmarkBase;
(function(BenchmarkBase) {
  ...
  class BenchmarkBase extends dart.Object { ... }
  ...

Running this triggers:

lib/src/DeltaBlue/dart/ddc/BenchmarkBase/BenchmarkBase.js:65: SyntaxError: Identifier 'BenchmarkBase' has already been declared

when the class is parsed by v8.

[jmesserly]

yeah hoping we can fix this and some related issues via #34: use ES6 modules and have a more sane desugaring

[jmesserly]

https://codereview.chromium.org/967713002/