Uplift to 2.5.2 (#455)

diff --git a/.travis.yml b/.travis.yml
index 833d11e..18f0c26 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,7 +3,7 @@
   - gem install coveralls-lcov
 dart:
   # Keep this value aligned with min SDK value in pubspec.yaml and FROM line of Dockerfile
-  - "2.5.1"
+  - "2.5.2"
   # TODO(devoncarew): commented out due to
   # https://github.com/dart-lang/dart-pad/issues/1002
   #- dev
diff --git a/Dockerfile b/Dockerfile
index 03532e3..b033ec0 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,5 +1,5 @@
 # Keep aligned with min SDK in pubspec.yaml and Dart test version in .travis.yml
-FROM google/dart:2.5.1
+FROM google/dart:2.5.2
 
 WORKDIR /app
 ADD tool/dart_run.sh /dart_runtime/
diff --git a/pubspec.lock b/pubspec.lock
index b942ad7..1396e1b 100644
--- a/pubspec.lock
+++ b/pubspec.lock
@@ -548,4 +548,4 @@
     source: hosted
     version: "2.1.16"
 sdks:
-  dart: ">=2.5.1 <3.0.0"
+  dart: "2.5.2"
diff --git a/pubspec.yaml b/pubspec.yaml
index dd89186..140f2d8 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -2,8 +2,8 @@
 publish_to: none
 
 environment:
-  # Keep min SDK aligned with .travis.yml and FROM line of Dockerfile
-  sdk: '>=2.5.1 <3.0.0'
+  # Keep SDK aligned with .travis.yml and FROM line of Dockerfile
+  sdk: 2.5.2
 
 dependencies:
   _discoveryapis_commons: 0.1.9