Small style tweaks to charts package that should not have a runtime impact. Make implementation cla…

Small style tweaks to charts package that should not have a runtime impact. Make implementation classes match the type signatures of base classes. Add a few generic types that weren't specified. Cleanup cases where a double was used as an index. Move super calls to end of constructor initialization list matching the Dart style guide. https://www.dartlang.org/articles/style-guide/#do-place-the-super-call-last-in-a-constructor-initialization-list

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[jacobr, 2015-01-23 22:22:51]

can you take a look and comment on changes you want me to remove om preparation
for future inference changes. This CL looks like it has a particularly large #
of cases that might be fixed by inference.
Once fixes you want me to remove are gone I'll send it off to owners for review.