Silence a bunch of annoying bogus dartanalyzer warnings.

Silence a bunch of annoying bogus dartanalyzer warnings.
 - several places where we use a getter knowing that it will return a
   subclass of its declared type, even asserting that it does, but
   where the analyzer is worried that the getter might start returning
   a new value unexpectedly, solved by having a temporary local
   variable shadow the getter and asserting that it doesn't change
   value
 - many many places where we do this with parentData specifically,
   solved by type-erasing parentData.
 - a place where a mixin wants to be subclassing another class, and
   uses its methods, solved by saying that the mixin is abstract but
   implements the superclass

R=abarth@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/0e6b06b7e39a2e80ee98371c7ad165fbd084c893

[abarth-chromium, 2015-06-05 16:40:50]

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

[abarth-chromium, 2015-06-05 16:40:51]

lgtm

[Hixie, 2015-06-05 16:44:13]

Committed patchset #1 (id:1) manually as
0e6b06b7e39a2e80ee98371c7ad165fbd084c893 (presubmit successful).