Implement type inference for null-aware property gets.

pkg/front_end/lib/src/fasta/kernel/frontend_accessors.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.
 
 /// A library to help transform compounds and null-aware accessors into
 /// let expressions.
 
 import 'package:front_end/src/fasta/kernel/kernel_shadow_ast.dart'
     show
         KernelArguments,
         KernelComplexAssignment,
         KernelConditionalExpression,
         KernelMethodInvocation,
+        KernelNullAwarePropertyGet,
         KernelPropertyAssign,
         KernelPropertyGet,
         KernelThisExpression,
         KernelVariableDeclaration,
         KernelVariableGet;
 
 import 'package:front_end/src/fasta/kernel/utils.dart' show offsetForToken;
 
 import 'package:front_end/src/scanner/token.dart' show Token;
 
@@ skipping 314 matching lines @@
   Expression _makeWrite(Expression value, bool voidContext,
       KernelComplexAssignment complexAssignment) {
     var write = new PropertySet(receiverAccess(), name, value, setter)
       ..fileOffset = offsetForToken(token);
     complexAssignment?.write = write;
     return write;
   }
 
   Expression _finish(
       Expression body, KernelComplexAssignment complexAssignment) {
-    var nullAwareGuard = new KernelConditionalExpression(
-        buildIsNull(receiverAccess(), offsetForToken(token)),
+    var offset = offsetForToken(token);
+    var nullAwareGuard = new ConditionalExpression(
+        buildIsNull(receiverAccess(), offset),
         new NullLiteral(),
-        body)
-      ..fileOffset = offsetForToken(token);
-    body = makeLet(receiver, nullAwareGuard);
+        body,
+        const DynamicType())
+      ..fileOffset = offset;
     if (complexAssignment != null) {
+      body = makeLet(receiver, nullAwareGuard);
       KernelPropertyAssign kernelPropertyAssign = complexAssignment;
       kernelPropertyAssign.nullAwareGuard = nullAwareGuard;
       kernelPropertyAssign.desugared = body;
       return kernelPropertyAssign;
     } else {
-      return body;
+      return new KernelNullAwarePropertyGet(receiver, nullAwareGuard)
+        ..fileOffset = offset;
     }
   }
 }
 
 class SuperPropertyAccessor extends Accessor {
   Name name;
   Member getter, setter;
 
   SuperPropertyAccessor(
       BuilderHelper helper, this.name, this.getter, this.setter, Token token)
@@ skipping 351 matching lines @@
 
 Expression buildIsNull(Expression value, int offset) {
   return makeBinary(value, equalsName, null, new NullLiteral(), offset: offset);
 }
 
 VariableDeclaration makeOrReuseVariable(Expression value) {
   // TODO: Devise a way to remember if a variable declaration was reused
   // or is fresh (hence needs a let binding).
   return new VariableDeclaration.forValue(value);
 }