precreate .dart directory to workaround #2412 (#2413)

* Disable travis to workaround #2412

* precreate .dart directory to work around #2412

* simplify workaround and add comment

Author: jcollins-g

tool/travis.sh

@@ -14,6 +14,8 @@ DART_VERSION=`dart --version 2>&1 | awk '{print $4}'`
 if ! echo "${DART_VERSION}" | grep -q dev || ! uname | grep -q Linux ; then
   unset COVERAGE_TOKEN
 fi
+# workaround for dart-lang/sdk#44027
+mkdir -p ~/.dart
 
 if [ "$DARTDOC_BOT" = "sdk-docs" ]; then
   # Build the SDK docs