Don't allow to extract a closure as a getter.

pkg/analysis_server/lib/src/services/refactoring/extract_method.dart

Index: pkg/analysis_server/lib/src/services/refactoring/extract_method.dart
diff --git a/pkg/analysis_server/lib/src/services/refactoring/extract_method.dart b/pkg/analysis_server/lib/src/services/refactoring/extract_method.dart
index 361d537c14e27bc73630659971c4b66f02ba8b7d..5126105cadde10631b06d882d69c1f75d9ae2080 100644
--- a/pkg/analysis_server/lib/src/services/refactoring/extract_method.dart
+++ b/pkg/analysis_server/lib/src/services/refactoring/extract_method.dart
@@ -71,6 +71,7 @@ class ExtractMethodRefactoringImpl extends RefactoringImpl
   String returnType;
   String name;
   bool extractAll = true;
+  bool canCreateGetter = false;
   bool createGetter = false;
   final List<String> names = <String>[];
   final List<int> offsets = <int>[];
@@ -100,18 +101,29 @@ class ExtractMethodRefactoringImpl extends RefactoringImpl
     utils = new CorrectionUtils(unit);
   }
 
-  bool get canCreateGetter {
+  /**
+   * Initializes [canCreateGetter] flag.
+   */
+  bool _computeCanCreateGetter() {
+    // is a function expression
+    if (_selectionFunctionExpression != null) {
+      return false;
+    }
+    // has parameters
     if (!parameters.isEmpty) {
       return false;
     }
+    // is assignment
     if (_selectionExpression != null) {
       if (_selectionExpression is AssignmentExpression) {
         return false;
       }
     }
+    // doesn't return a value
     if (_selectionStatements != null) {
       return returnType != 'void';
     }
+    // OK
     return true;
   }
 
@@ -188,10 +200,12 @@ class ExtractMethodRefactoringImpl extends RefactoringImpl
     // prepare parts
     result.addStatus(_initializeParameters());
     _initializeReturnType();
-    _initializeGetter();
     // occurrences
     _initializeOccurrences();
     _prepareOffsetsLengths();
+    // getter
+    canCreateGetter = _computeCanCreateGetter();
+    _initializeCreateGetter();
     // names
     _prepareExcludedNames();
     _prepareNames();
@@ -511,7 +525,7 @@ class ExtractMethodRefactoringImpl extends RefactoringImpl
   /**
    * Initializes [createGetter] flag.
    */
-  void _initializeGetter() {
+  void _initializeCreateGetter() {
     createGetter = false;
     // maybe we cannot at all
     if (!canCreateGetter) {