Emit a compile-time error when returning a value from a constructor.

sdk/lib/_internal/compiler/implementation/warnings.dart

Index: sdk/lib/_internal/compiler/implementation/warnings.dart
diff --git a/sdk/lib/_internal/compiler/implementation/warnings.dart b/sdk/lib/_internal/compiler/implementation/warnings.dart
index 7e8a8d344dba86b57997f909fe187d59d781063a..59055aead79bb4fed3df30f75f7db6944ff659df 100644
--- a/sdk/lib/_internal/compiler/implementation/warnings.dart
+++ b/sdk/lib/_internal/compiler/implementation/warnings.dart
@@ -500,6 +500,18 @@ class MessageKind {
   static const MessageKind CONSTRUCTOR_WITH_RETURN_TYPE = const MessageKind(
       'Error: Cannot have return type for constructor.');
 
+  static const MessageKind CANNOT_RETURN_FROM_CONSTRUCTOR = const MessageKind(
+      "Error: Cannot return a value from a constructor.",

[ahe, 2013/09/05 08:35:20]

Prefer "can't" over "cannot" (the guidelines were updated based on feedback from
usability experts and tech writers).

+      howToFix: "Remove the return statement or use a factory constructor.",
+      examples: const ["""
+class C {
+  C() {
+    return 1;
+  }
+}
+
+main() => new C();"""]);
+
   static const MessageKind ILLEGAL_FINAL_METHOD_MODIFIER = const MessageKind(
       'Error: Cannot have final modifier on method.');