Add some type inference for "yield" statements.

pkg/front_end/lib/src/fasta/kernel/body_builder.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.
 
 library fasta.body_builder;
 
 import '../fasta_codes.dart'
     show FastaMessage, codeExpectedButGot, codeExpectedFunctionBody;
 
 import '../parser/parser.dart' show FormalParameterType, MemberKind, optional;
 
 import '../parser/identifier_context.dart' show IdentifierContext;
 
 import 'package:front_end/src/fasta/builder/ast_factory.dart' show AstFactory;
 
 import 'package:front_end/src/fasta/kernel/kernel_shadow_ast.dart'
     show
         KernelArguments,
         KernelField,
         KernelFunctionDeclaration,
-        KernelReturnStatement;
+        KernelReturnStatement,
+        KernelYieldStatement;
 
 import 'package:front_end/src/fasta/kernel/utils.dart' show offsetForToken;
 
 import 'package:front_end/src/fasta/type_inference/type_inference_engine.dart'
     show FieldNode;
 
 import 'package:front_end/src/fasta/type_inference/type_inferrer.dart'
     show TypeInferrer;
 
 import 'package:front_end/src/fasta/type_inference/type_promotion.dart'
@@ skipping 2279 matching lines @@
       Token commaToken, Token rightParenthesis, Token semicolonToken) {
     debugEvent("AssertStatement");
     Expression message = popForValueIfNotNull(commaToken);
     Expression condition = popForValue();
     push(new AssertStatement(condition, message));
   }
 
   @override
   void endYieldStatement(Token yieldToken, Token starToken, Token endToken) {
     debugEvent("YieldStatement");
-    push(new YieldStatement(popForValue(), isYieldStar: starToken != null)
+    push(new KernelYieldStatement(popForValue(), isYieldStar: starToken != null)
       ..fileOffset = yieldToken.charOffset);
   }
 
   @override
   void beginSwitchBlock(Token token) {
     debugEvent("beginSwitchBlock");
     enterLocalScope();
     enterSwitchScope();
     enterBreakTarget(token.charOffset);
   }
@@ skipping 834 matching lines @@
     if (starToken == null) {
       return AsyncMarker.Async;
     } else {
       assert(identical(starToken.stringValue, "*"));
       return AsyncMarker.AsyncStar;
     }
   } else {
     return internalError("Unknown async modifier: $asyncToken");
   }
 }