Use putIfAbsent in InheritanceManager._reportError.

pkg/analyzer/lib/src/dart/resolver/inheritance_manager.dart

Index: pkg/analyzer/lib/src/dart/resolver/inheritance_manager.dart
diff --git a/pkg/analyzer/lib/src/dart/resolver/inheritance_manager.dart b/pkg/analyzer/lib/src/dart/resolver/inheritance_manager.dart
index ba9d1ff9684dba3a617a075a45e421f5347ed34a..69d3948ded0b4ec95e4711eca38ec0a01b067572 100644
--- a/pkg/analyzer/lib/src/dart/resolver/inheritance_manager.dart
+++ b/pkg/analyzer/lib/src/dart/resolver/inheritance_manager.dart
@@ -669,11 +669,8 @@ class InheritanceManager {
     if (ignoreErrors) {
       return;
     }
-    HashSet<AnalysisError> errorSet = _errorsInClassElement[classElt];
-    if (errorSet == null) {
-      errorSet = new HashSet<AnalysisError>();
-      _errorsInClassElement[classElt] = errorSet;
-    }
+    HashSet<AnalysisError> errorSet = _errorsInClassElement.putIfAbsent(
+        classElt, () => new HashSet<AnalysisError>());
     errorSet.add(new AnalysisError(classElt.source, classElt.nameOffset,
         classElt.nameLength, errorCode, arguments));
   }