Update yield warnings.

pkg/compiler/lib/src/typechecker.dart

 // Copyright (c) 2012, 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.
 
 part of dart2js;
 
 class TypeCheckerTask extends CompilerTask {
   TypeCheckerTask(Compiler compiler) : super(compiler);
   String get name => "Type checker";
 
@@ skipping 1611 matching lines @@
 
   DartType visitAwait(Await node) {
     DartType expressionType = analyze(node.expression);
     return flatten(expressionType);
   }
 
   DartType visitYield(Yield node) {
     DartType resultType = analyze(node.expression);
     if (!node.hasStar) {
       if (currentAsyncMarker.isAsync) {
-        resultType =
-            compiler.streamClass.thisType.createInstantiation(
-                <DartType>[resultType]);
+        resultType = compiler.coreTypes.streamType(resultType);
       } else {
-        resultType =
-            compiler.iterableClass.thisType.createInstantiation(
-                 <DartType>[resultType]);
+        resultType = compiler.coreTypes.iterableType(resultType);
+      }
+    } else {
+      if (currentAsyncMarker.isAsync) {
+        // The static type of expression must be assignable to Stream.
+        checkAssignable(node, resultType, compiler.coreTypes.streamType());
+      } else {
+        // The static type of expression must be assignable to Iterable.
+        checkAssignable(node, resultType, compiler.coreTypes.iterableType());
       }
     }
+    // The static type of the result must be assignable to the declared type.
     checkAssignable(node, resultType, expectedReturnType);
     return const StatementType();
   }
 
   DartType visitTypeAnnotation(TypeAnnotation node) {
     return elements.getType(node);
   }
 
   DartType visitVariableDefinitions(VariableDefinitions node) {
     DartType type = analyzeWithDefault(node.type, const DynamicType());
@@ skipping 188 matching lines @@
 
   visitTypedef(Typedef node) {
     // Do not typecheck [Typedef] nodes.
   }
 
   visitNode(Node node) {
     compiler.internalError(node,
         'Unexpected node ${node.getObjectDescription()} in the type checker.');
   }
 }