Adds typechecking of return values

frog/gen.dart

Index: frog/gen.dart
diff --git a/frog/gen.dart b/frog/gen.dart
index 50f81faefccc8ddf16ed1f90ecdecc1c7f503db8..834a3b8722ebee0771624c0fe905aa72d9b87187 100644
--- a/frog/gen.dart
+++ b/frog/gen.dart
@@ -1100,12 +1100,15 @@ class MethodGenerator implements TreeVisitor {
    */
   bool visitReturnStatement(ReturnStatement node) {
     if (node.value == null) {
+      // This is essentially "return null".
+      // It can't issue a warning because every type is nullable.
       writer.writeln('return;');
     } else {
       if (method.isConstructor) {
         world.error('return of value not allowed from constructor', node.span);
       }
-      writer.writeln('return ${visitValue(node.value).code};');
+      var value = visitTypedValue(node.value, method.returnType);
+      writer.writeln('return ${value.code};');
     }
     return true;
   }