ValueTrend, RollupTrend, Trend, Context

- Context is implemented as an object in util.js. require() is pretty smart about handling multiple instances of the same file, so we can use it to create a 'global' context.
- Trend is a helper class now, not a base class. It is more or less the same as the Trends class implemented in the Dart SDK. ValueTrend/RollupTrend rely on duck typing to be both 'instances' of Trend, trying to do an interface-like class for Trend really wasn't working out.
- RollupTrend and ValueTrend work as expected. I fixed multiple bugs in the Dart SDK I encountered while porting it over, and notified @kaendfinger about them.
- TimeRange, works as expected.
Remove unused require.