Bump min Dart SDK to 2.7 (dart-lang/pubspec_parse#59)

Works-around issue with testing Dart 2.6 in CI and changes to pkg:analyzer
diff --git a/pkgs/pubspec_parse/.travis.yml b/pkgs/pubspec_parse/.travis.yml
index 66bff3d..802491f 100644
--- a/pkgs/pubspec_parse/.travis.yml
+++ b/pkgs/pubspec_parse/.travis.yml
@@ -1,7 +1,7 @@
 language: dart
 
 dart:
-  - 2.6.0
+  - 2.7.0
   - dev
 
 dart_task:
@@ -16,7 +16,7 @@
     - dart: dev
       dart_task:
         dartanalyzer: --fatal-infos --fatal-warnings .
-    - dart: 2.6.0
+    - dart: 2.7.0
       dart_task:
         dartanalyzer: --fatal-warnings .
 
diff --git a/pkgs/pubspec_parse/CHANGELOG.md b/pkgs/pubspec_parse/CHANGELOG.md
index df5c0e8..7e74a55 100644
--- a/pkgs/pubspec_parse/CHANGELOG.md
+++ b/pkgs/pubspec_parse/CHANGELOG.md
@@ -1,6 +1,6 @@
 ## 0.1.6-dev
 
-- Update SDK requirement to `>=2.6.0 <3.0.0`.
+- Update SDK requirement to `>=2.7.0 <3.0.0`.
 
 ## 0.1.5
 
diff --git a/pkgs/pubspec_parse/pubspec.yaml b/pkgs/pubspec_parse/pubspec.yaml
index 2940ce3..09d2fb9 100644
--- a/pkgs/pubspec_parse/pubspec.yaml
+++ b/pkgs/pubspec_parse/pubspec.yaml
@@ -6,7 +6,7 @@
 homepage: https://github.com/dart-lang/pubspec_parse
 
 environment:
-  sdk: '>=2.6.0 <3.0.0'
+  sdk: '>=2.7.0 <3.0.0'
 
 dependencies:
   checked_yaml: ^1.0.0