Add some type inference for "yield" statements.

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 800 matching lines @@
         ? closureContext.returnContext
         : null;
     var inferredType = expression != null
         ? inferrer.inferExpression(
             expression, typeContext, closureContext != null)
         : const VoidType();
     // Analyzer treats bare `return` statements as having no effect on the
     // inferred type of the closure.  TODO(paulberry): is this what we want
     // for Fasta?
     if (expression != null) {
-      closureContext?.updateInferredReturnType(inferrer, inferredType);
+      closureContext?.handleReturn(inferrer, inferredType);
     }
     inferrer.listener.returnStatementExit(this);
   }
 }
 
 /// Common base class for shadow objects representing statements in kernel
 /// form.
 abstract class KernelStatement extends Statement {
   /// Calls back to [inferrer] to perform type inference for whatever concrete
   /// type of [KernelStatement] this is.
@@ skipping 505 matching lines @@
       KernelTypeInferrer inferrer, DartType typeContext, bool typeNeeded) {
     var variable = this.variable as KernelVariableDeclaration;
     typeNeeded =
         inferrer.listener.variableSetEnter(this, typeContext) || typeNeeded;
     var inferredType =
         inferrer.inferExpression(value, variable._declaredType, typeNeeded);
     inferrer.listener.variableSetExit(this, inferredType);
     return inferredType;
   }
 }
+
+/// Concrete shadow object representing a yield statement in kernel form.
+class KernelYieldStatement extends YieldStatement implements KernelStatement {
+  KernelYieldStatement(Expression expression, {bool isYieldStar: false})
+      : super(expression, isYieldStar: isYieldStar);
+
+  @override
+  void _inferStatement(KernelTypeInferrer inferrer) {
+    inferrer.listener.yieldStatementEnter(this);
+    var closureContext = inferrer.closureContext;
+    var typeContext = closureContext != null && closureContext.isGenerator
+        ? closureContext.returnContext
+        : null;
+    if (isYieldStar && typeContext != null) {
+      if (closureContext == null) {
+        typeContext = null;
+      } else {
+        typeContext = inferrer.wrapType(
+            typeContext,
+            closureContext.isAsync
+                ? inferrer.coreTypes.streamClass
+                : inferrer.coreTypes.iterableClass);
+      }
+    }
+    var inferredType = inferrer.inferExpression(
+        expression, typeContext, closureContext != null);
+    closureContext?.handleYield(inferrer, isYieldStar, inferredType);
+    inferrer.listener.yieldStatementExit(this);
+  }
+}