refactor/simplify nullable inference code

lib/src/codegen/side_effect_analysis.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/constant.dart';
 import 'package:analyzer/src/generated/element.dart';
-import 'package:analyzer/src/generated/error.dart' show ErrorReporter;
-import 'package:analyzer/src/generated/engine.dart' show RecordingErrorListener;
+import 'package:analyzer/src/generated/error.dart'
+    show AnalysisErrorListener, ErrorReporter;
 import 'package:analyzer/src/generated/resolver.dart' show TypeProvider;
+import 'package:analyzer/src/generated/source.dart' show Source;
 
 /// True is the expression can be evaluated multiple times without causing
 /// code execution. This is true for final fields. This can be true for local
 /// variables, if:
 /// * they are not assigned within the [context].
 /// * they are not assigned in a function closure anywhere.
 /// True is the expression can be evaluated multiple times without causing
 /// code execution. This is true for final fields. This can be true for local
 /// variables, if:
 ///
@@ skipping 69 matching lines @@
 
     if (node.inSetterContext() && node.staticElement == _variable) {
       _potentiallyMutated = true;
     }
   }
 }
 
 class ConstFieldVisitor {
   final ConstantVisitor _constantVisitor;
 
-  ConstFieldVisitor(TypeProvider types, CompilationUnit unit)
+  ConstFieldVisitor(TypeProvider types, Source source)
       // TODO(jmesserly): support -D variables on the command line
       : _constantVisitor = new ConstantVisitor(
             new ConstantEvaluationEngine(types, new DeclaredVariables()),
-            new ErrorReporter(
-                new RecordingErrorListener(), unit.element.source));
+            new ErrorReporter(AnalysisErrorListener.NULL_LISTENER, source));
 
   // TODO(jmesserly): this is used to determine if the field initialization is
   // side effect free. We should make the check more general, as things like
   // list/map literals/regexp are also side effect free and fairly common
   // to use as field initializers.
   bool isFieldInitConstant(VariableDeclaration field) =>
       field.initializer == null || computeConstant(field) != null;
 
   DartObject computeConstant(VariableDeclaration field) {
     // If the constant is already computed by ConstantEvaluator, just return it.
     VariableElementImpl element = field.element;
     var result = element.evaluationResult;
     if (result != null) return result.value;
 
     // ConstantEvaluator will not compute constants for non-const fields,
     // so run ConstantVisitor for those to figure out if the initializer is
     // actually a constant (and therefore side effect free to evaluate).
     assert(!field.isConst);
 
     var initializer = field.initializer;
     if (initializer == null) return null;
     return initializer.accept(_constantVisitor);
   }
 }