add travis support

.travis.yml

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+language: dart
+sudo: false
+dart:
+  - dev
+  - stable
+script: ./tool/travis.sh

tool/ensure_dartfmt.sh

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+#!/bin/sh
+
+dart_files=$(git ls-tree --name-only --full-tree -r HEAD | grep '.dart$')
+[ -z "$dart_files" ] && exit 0
+
+unformatted=$(dartfmt -n $dart_files)
+[ -z "$unformatted" ] && exit 0
+
+# Some files are not dartfmt'd. Print message and fail.
+echo >&2 "dart files must be formatted with dartfmt. Please run:"
+for fn in $unformatted; do
+  echo >&2 "  dartfmt -w $PWD/$fn"
+done
+
+exit 1

tool/travis.sh

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+#!/bin/bash
+
+# Fast fail the script on failures.   
+set -e
+
+$(dirname -- "$0")/ensure_dartfmt.sh
+
+# Run the tests.
+dart --checked test/test_all.dart
+
+# Install dart_coveralls; gather and send coverage data.
+if [ "$COVERALLS_TOKEN" ] && [ "$TRAVIS_DART_VERSION" = "stable" ]; then
+  pub global activate dart_coveralls
+  pub global run dart_coveralls report \
+    --token $COVERALLS_TOKEN \
+    --retry 2 \
+    --exclude-test-files \
+    test/test_all.dart
+fi