Fix handling of null/bottom in front_end type inference.

pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart

 // Copyright (c) 2017, 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.
 
 /// This file declares a "shadow hierarchy" of concrete classes which extend
 /// the kernel class hierarchy, adding methods and fields needed by the
 /// BodyBuilder.
 ///
 /// Instances of these classes may be created using the factory methods in
 /// `ast_factory.dart`.
@@ skipping 346 matching lines @@
       : super(receiver, name, arguments, interfaceTarget);
 
   KernelMethodInvocation.byReference(Expression receiver, Name name,
       Arguments arguments, Reference interfaceTargetReference)
       : super.byReference(receiver, name, arguments, interfaceTargetReference);
 
   @override
   DartType _inferExpression(
       KernelTypeInferrer inferrer, DartType typeContext, bool typeNeeded) {
     // TODO(scheglov): implement.
+    inferrer.inferExpression(receiver, null, false);
     return typeNeeded ? const DynamicType() : null;
   }
 }
 
 /// Shadow object for [Not].
 class KernelNot extends Not implements KernelExpression {
   KernelNot(Expression operand) : super(operand);
 
   @override
   DartType _inferExpression(
       KernelTypeInferrer inferrer, DartType typeContext, bool typeNeeded) {
     // TODO(scheglov): implement.
     return typeNeeded ? const DynamicType() : null;
   }
 }
 
 /// Concrete shadow object representing a null literal in kernel form.
 class KernelNullLiteral extends NullLiteral implements KernelExpression {
   @override
   DartType _inferExpression(
       KernelTypeInferrer inferrer, DartType typeContext, bool typeNeeded) {
-    // TODO(paulberry): implement.
-    return typeNeeded ? const DynamicType() : null;
+    return inferrer.inferNullLiteral(typeContext, typeNeeded);
   }
 }
 
 /// Shadow object for [PropertyGet].
 class KernelPropertyGet extends PropertyGet implements KernelExpression {
   KernelPropertyGet(Expression receiver, Name name, [Member interfaceTarget])
       : super(receiver, name, interfaceTarget);
 
   KernelPropertyGet.byReference(
       Expression receiver, Name name, Reference interfaceTargetReference)
@@ skipping 488 matching lines @@
       : super(variable, value);
 
   @override
   DartType _inferExpression(
       KernelTypeInferrer inferrer, DartType typeContext, bool typeNeeded) {
     var variable = this.variable as KernelVariableDeclaration;
     return inferrer.inferVariableSet(
         typeContext, typeNeeded, variable._declaredType, value);
   }
 }