Enable downward inference on default params

lib/src/checker/checker.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.
 
 library dev_compiler.src.checker.checker;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/scanner.dart' show Token, TokenType;
@@ skipping 347 matching lines @@
     _rules.reportMissingType = callback;
   }
 
   _visitMaybeConst(AstNode n, visitNode(AstNode n)) {
     var o = _constantContext;
     if (!o) {
       if (n is VariableDeclarationList) {
         _constantContext = o || n.isConst;
       } else if (n is VariableDeclaration) {
         _constantContext = o || n.isConst;
+      } else if (n is DefaultFormalParameter) {
+        _constantContext = true;
       } else if (n is FormalParameter) {
         _constantContext = o || n.isConst;
       } else if (n is InstanceCreationExpression) {
         _constantContext = o || n.isConst;
       } else if (n is ConstructorDeclaration) {
         _constantContext = o || n.element.isConst;
       }
     }
     visitNode(n);
     _constantContext = o;
@@ skipping 213 matching lines @@
     node.visitChildren(this);
   }
 
   @override visitDefaultFormalParameter(DefaultFormalParameter node) {
     _visitMaybeConst(node, (node) {
       // Check that defaults have the proper subtype.
       var parameter = node.parameter;
       var parameterType = _rules.elementType(parameter.element);
       assert(parameterType != null);
       var defaultValue = node.defaultValue;
-      var defaultType;
       if (defaultValue == null) {
-        // TODO(vsm): Should this be null?
-        defaultType = _rules.provider.bottomType;
+        if (_rules.maybeNonNullableType(parameterType)) {
+          var staticInfo = new InvalidVariableDeclaration(
+              _rules, node.identifier, parameterType);
+          _recordMessage(staticInfo);
+        }
       } else {
-        defaultType = _rules.getStaticType(defaultValue);
+        var staticInfo = checkAssignment(defaultValue, parameterType);
+        if (staticInfo is! StaticError) node.defaultValue = staticInfo;
       }
 
-      // If defaultType is bottom, this enforces that parameterType is not
-      // non-nullable.
-      if (!_rules.isSubTypeOf(defaultType, parameterType)) {
-        var staticInfo = (defaultValue == null)
-            ? new InvalidVariableDeclaration(
-                _rules, node.identifier, parameterType)
-            : new StaticTypeError(_rules, defaultValue, parameterType);
-        _recordMessage(staticInfo);
-      }
       node.visitChildren(this);
     });
   }
 
   visitFieldFormalParameter(FieldFormalParameter node) {
     var element = node.element;
     var typeName = node.type;
     if (typeName != null) {
       var type = _rules.elementType(element);
       var fieldElement =
@@ skipping 173 matching lines @@
   void _recordDynamicInvoke(AstNode node) {
     _reporter.log(new DynamicInvoke(_rules, node));
   }
 
   void _recordMessage(StaticInfo info) {
     if (info == null) return;
     if (info.level >= logger.Level.SEVERE) _failure = true;
     _reporter.log(info);
   }
 }