This CL begins removing gyp from Dart's tree.
Removing gyp will go in three stages:
1. Remove unused .gyp and .gypi files.
2. Translate *_sources.gypi files to *_sources.gni files
3. Remove //third_party/gyp from DEPS
This CL is stage 1.
R=asiva@google.com
Review-Url: https://codereview.chromium.org/2992593002 .