Temporarily pin Travis Dart SDK to 2.0.0-dev.63.0 (flutter#5586)

This is a temporary workaround to unbreak the build. The breakage was
caused by differences in the latest released dev Dart SDK
(2.0.0-dev.64.0) and the version currently used by Flutter
(2.0.0-dev.63.0).

Revert once the SDK has been updated in the engine.

.travis.yml

@@ -1,6 +1,6 @@
 language: dart
 dart:
-  - dev
+  - "dev/release/2.0.0-dev.63.0"
 sudo: false
 before_script:
   - ./travis/setup.sh