Implement type inference for symbol literals.

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 1850 matching lines @@
   KernelSymbolLiteral(String value) : super(value);
 
   @override
   void _collectDependencies(KernelDependencyCollector collector) {
     // No inference dependencies.
   }
 
   @override
   DartType _inferExpression(
       KernelTypeInferrer inferrer, DartType typeContext, bool typeNeeded) {
-    // TODO(scheglov): implement.
-    return typeNeeded ? const DynamicType() : null;
+    typeNeeded =
+        inferrer.listener.symbolLiteralEnter(this, typeContext) || typeNeeded;
+    var inferredType =
+        typeNeeded ? inferrer.coreTypes.symbolClass.rawType : null;
+    inferrer.listener.symbolLiteralExit(this, inferredType);
+    return inferredType;
   }
 }
 
 /// Shadow object for [ThisExpression].
 class KernelThisExpression extends ThisExpression implements KernelExpression {
   @override
   void _collectDependencies(KernelDependencyCollector collector) {
     // Field initializers are not allowed to refer to [this].  But if it
     // happens, we can still proceed; no additional type inference dependencies
     // are introduced.
@@ skipping 441 matching lines @@
   }
 
   transformChildren(v) {
     return internalError("Internal error: Unsupported operation.");
   }
 
   visitChildren(v) {
     return internalError("Internal error: Unsupported operation.");
   }
 }