Add some type inference for "yield" statements.

pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart

Index: pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart
diff --git a/pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart b/pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart
index d6258de42710abf3ac131dd6e07f1cf4fd44cb6e..b74bc4c6cccf2d4a9fb921ed36161ea552996418 100644
--- a/pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart
+++ b/pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart
@@ -51,7 +51,28 @@ class ClosureContext {
 
   /// Updates the inferred return type based on the presence of a return
   /// statement returning the given [type].
-  void updateInferredReturnType(TypeInferrerImpl inferrer, DartType type) {
+  void handleReturn(TypeInferrerImpl inferrer, DartType type) {
+    if (isGenerator) return;
+    if (isAsync) {
+      type = inferrer.typeSchemaEnvironment.flattenFutures(type);
+    }
+    _updateInferredReturnType(inferrer, type);
+  }
+
+  void handleYield(TypeInferrerImpl inferrer, bool isYieldStar, DartType type) {
+    if (!isGenerator) return;
+    if (isYieldStar) {
+      type = inferrer.getTypeArgumentOf(
+          type,
+          isAsync
+              ? inferrer.coreTypes.streamClass
+              : inferrer.coreTypes.iterableClass);
+      if (type == null) return;
+    }
+    _updateInferredReturnType(inferrer, type);
+  }
+
+  void _updateInferredReturnType(TypeInferrerImpl inferrer, DartType type) {
     if (_inferredReturnType == null) {
       _inferredReturnType = type;
     } else {
@@ -194,6 +215,17 @@ abstract class TypeInferrerImpl extends TypeInferrer {
     }
   }
 
+  DartType getDerivedTypeArgumentOf(DartType type, Class class_) {
+    if (type is InterfaceType) {
+      var typeAsInstanceOfClass =
+          classHierarchy.getTypeAsInstanceOf(type, class_);
+      if (typeAsInstanceOfClass != null) {
+        return typeAsInstanceOfClass.typeArguments[0];
+      }
+    }
+    return null;
+  }
+
   /// Modifies a type as appropriate when inferring a declared variable's type.
   DartType inferDeclarationType(DartType initializerType) {
     if (initializerType is BottomType ||