Remove Material's opinion about background colours, since in fact the background colour varies dram…

Remove Material's opinion about background colours, since in fact the background colour varies dramatically with the kind of material involved.

Also:
- give users of Material who previously relied on Material's defaults
  opinions about background colours
- reindent stock_row.dart
- import colors as colors in example/widgets/styled_text.dart, rather
  than in global scope, for consistency with other files
- import typography as typograpyh in example/widgets/tabs.dart, rather
  than in global scope, for consistency with other files
- make flat_button actually be transparent by not painting a
  background, rather than painting 0x00000000. This is the what I
  actually originally set out to do here.

R=abarth@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/a81bf4f2696f51c1b6ced736c2a43127f6c787c4

[abarth-chromium, 2015-06-30 19:47:35]

abarth@chromium.org changed reviewers:
+ abarth@chromium.org

[abarth-chromium, 2015-06-30 19:47:36]

lgtm

[Hixie, 2015-06-30 19:48:09]

Committed patchset #2 (id:20001) manually as
a81bf4f2696f51c1b6ced736c2a43127f6c787c4 (presubmit successful).