don't produce errors from inference for downward inferred type variables

pkg/analyzer/test/generated/strong_mode_test.dart

 // Copyright (c) 2016, 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 analyzer.test.generated.strong_mode_test;
 
 import 'dart:async';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_resolution_map.dart';
@@ skipping 1643 matching lines @@
     String code = r'''
     class A<S, T> {
       S s;
       T t;
     }
     class B<S> extends A<S, S> { B(S s); }
     A<int, String> test() => new B(3);
    ''';
     Source source = addSource(code);
     TestAnalysisResult analysisResult = await computeAnalysisResult(source);
-    assertErrors(source,
-        [StrongModeCode.COULD_NOT_INFER, StrongModeCode.INVALID_CAST_LITERAL]);
+    assertErrors(source,[StrongModeCode.INVALID_CAST_LITERAL]);
     verify([source]);
     CompilationUnit unit = analysisResult.unit;
     FunctionDeclaration test = AstFinder.getTopLevelFunction(unit, "test");
     ExpressionFunctionBody body = test.functionExpression.body;
     DartType type = body.expression.staticType;
 
     Element elementB = AstFinder.getClass(unit, "B").element;
 
     _isInstantiationOf(_hasElement(elementB))([_isNull])(type);
   }
@@ skipping 31 matching lines @@
     class A<S, T> {
       S s;
       T t;
     }
     class B<S> extends A<S, S> { B(S s); }
     A<int, double> test() => new B(3);
    ''';
     Source source = addSource(code);
     TestAnalysisResult analysisResult = await computeAnalysisResult(source);
     assertErrors(source, [
-      StrongModeCode.COULD_NOT_INFER,
       StrongModeCode.INVALID_CAST_LITERAL,
     ]);
     verify([source]);
     CompilationUnit unit = analysisResult.unit;
     FunctionDeclaration test = AstFinder.getTopLevelFunction(unit, "test");
     ExpressionFunctionBody body = test.functionExpression.body;
     DartType type = body.expression.staticType;
 
     Element elementB = AstFinder.getClass(unit, "B").element;
 
@@ skipping 1889 matching lines @@
   var v = x;
   v; // marker
 }
 int x = 3;
 ''';
     CompilationUnit unit = await resolveSource(code);
     assertPropagatedAssignedType(code, unit, typeProvider.intType, null);
     assertTypeOfMarkedExpression(code, unit, typeProvider.intType, null);
   }
 }