enable lint prefer_adjacent_string_concatenation (#9493)

.analysis_options

@@ -90,7 +90,7 @@ linter:
     - package_api_docs
     - package_prefixed_library_names
     # - parameter_assignments # we do this commonly
-    # - prefer_adjacent_string_concatenation # not yet tested
+    - prefer_adjacent_string_concatenation
     - prefer_collection_literals
     - prefer_const_constructors
     - prefer_contains

.analysis_options_repo

@@ -88,7 +88,7 @@ linter:
     - package_api_docs
     - package_prefixed_library_names
     # - parameter_assignments # we do this commonly
-    # - prefer_adjacent_string_concatenation # not yet tested
+    - prefer_adjacent_string_concatenation
     - prefer_collection_literals
     - prefer_const_constructors
     - prefer_contains

packages/flutter/lib/src/widgets/dismissible.dart

@@ -377,8 +377,8 @@ class _DismissibleState extends State<Dismissible> with TickerProviderStateMixin
         if (_resizeAnimation.status != AnimationStatus.forward) {
           assert(_resizeAnimation.status == AnimationStatus.completed);
           throw new FlutterError(
-            'A dismissed Dismissible widget is still part of the tree.\n' +
-            'Make sure to implement the onDismissed handler and to immediately remove the Dismissible\n' +
+            'A dismissed Dismissible widget is still part of the tree.\n'
+            'Make sure to implement the onDismissed handler and to immediately remove the Dismissible\n'
             'widget from the application once that handler has fired.'
           );
         }

packages/flutter/test/material/text_field_test.dart

@@ -40,8 +40,8 @@ void main() {
   SystemChannels.platform.setMockMethodCallHandler(mockClipboard.handleMethodCall);
 
   const String kThreeLines =
-    'First line of text is ' +
-    'Second line goes until ' +
+    'First line of text is '
+    'Second line goes until '
     'Third line of stuff ';
   const String kFourLines =
     kThreeLines +