Typecheck return with respect to async.

pkg/compiler/lib/src/resolution/members.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 resolution;
 
 abstract class TreeElements {
   AnalyzableElement get analyzedElement;
   Iterable<Node> get superUses;
 
@@ skipping 502 matching lines @@
             MessageKind.ASYNC_MODIFIER_ON_CONSTRUCTOR,
             {'modifier': element.asyncMarker});
       } else {
         if (element.isSetter) {
           compiler.reportError(asyncModifier,
               MessageKind.ASYNC_MODIFIER_ON_SETTER,
               {'modifier': element.asyncMarker});
 
         }
         if (functionExpression.body.asReturn() != null &&
-                 element.asyncMarker.isYielding) {
+            element.asyncMarker.isYielding) {
           compiler.reportError(asyncModifier,
               MessageKind.YIELDING_MODIFIER_ON_ARROW_BODY,
               {'modifier': element.asyncMarker});
         }
       }
       registry.registerAsyncMarker(element);
+      switch (element.asyncMarker) {
+      case AsyncMarker.ASYNC:
+        compiler.futureClass.ensureResolved(compiler);
+        break;
+      case AsyncMarker.ASYNC_STAR:
+        compiler.streamClass.ensureResolved(compiler);
+        break;
+      case AsyncMarker.SYNC_STAR:
+        compiler.iterableClass.ensureResolved(compiler);
+        break;
+      }
     }
   }
 
   bool _isNativeClassOrExtendsNativeClass(ClassElement classElement) {
     assert(classElement != null);
     while (classElement != null) {
       if (classElement.isNative) return true;
       classElement = classElement.superclass;
     }
     return false;
@@ skipping 2585 matching lines @@
   }
 
   visitRethrow(Rethrow node) {
     if (!inCatchBlock) {
       error(node, MessageKind.THROW_WITHOUT_EXPRESSION);
     }
   }
 
   visitReturn(Return node) {
     Node expression = node.expression;
-    if (expression != null &&
-        enclosingElement.isGenerativeConstructor) {
-      // It is a compile-time error if a return statement of the form
-      // `return e;` appears in a generative constructor.  (Dart Language
-      // Specification 13.12.)
-      compiler.reportError(expression,
-                           MessageKind.CANNOT_RETURN_FROM_CONSTRUCTOR);
+    if (expression != null) {
+      if (enclosingElement.isGenerativeConstructor) {
+        // It is a compile-time error if a return statement of the form
+        // `return e;` appears in a generative constructor.  (Dart Language
+        // Specification 13.12.)
+        compiler.reportError(expression,
+                             MessageKind.CANNOT_RETURN_FROM_CONSTRUCTOR);
+      } else if (!node.isArrowBody && currentAsyncMarker.isYielding) {
+        compiler.reportError(
+            node, 
+            MessageKind.RETURN_IN_GENERATOR, 
+            {'modifier': currentAsyncMarker});
+      }
     }
     visit(node.expression);
   }
 
   visitYield(Yield node) {
     compiler.streamClass.ensureResolved(compiler);
     compiler.iterableClass.ensureResolved(compiler);
     visit(node.expression);
   }
 
@@ skipping 1897 matching lines @@
 }
 
 /// The result for the resolution of the `assert` method.
 class AssertResult implements ResolutionResult {
   const AssertResult();
 
   Element get element => null;
 
   String toString() => 'AssertResult()';
 }