Allowing `e` of any type in `void f(...) => e` in dart2js.

pkg/compiler/lib/src/typechecker.dart

Index: pkg/compiler/lib/src/typechecker.dart
diff --git a/pkg/compiler/lib/src/typechecker.dart b/pkg/compiler/lib/src/typechecker.dart
index cbb5c246a66d1ee357fc11a5a80400aed921869d..ca3ef3fe1b424bd54886fd99c01c3d4d4c110496 100644
--- a/pkg/compiler/lib/src/typechecker.dart
+++ b/pkg/compiler/lib/src/typechecker.dart
@@ -1707,7 +1707,10 @@ class TypeCheckerVisitor extends Visitor<ResolutionDartType> {
         }
         if (expectedReturnType.isVoid &&
             !types.isAssignable(expressionType, const ResolutionVoidType())) {
-          reportTypeWarning(expression, MessageKind.RETURN_VALUE_IN_VOID);
+          // In `void f(...) => e`, `e` can have any type.
+          if (!node.isArrowBody) {
+            reportTypeWarning(expression, MessageKind.RETURN_VALUE_IN_VOID);
+          }
         } else {
           checkAssignable(expression, expressionType, expectedReturnType);
         }