Implement checked mode for const constructors.

lib/compiler/implementation/js_backend/constant_system_javascript.dart

Index: lib/compiler/implementation/js_backend/constant_system_javascript.dart
===================================================================
--- lib/compiler/implementation/js_backend/constant_system_javascript.dart	(revision 12329)
+++ lib/compiler/implementation/js_backend/constant_system_javascript.dart	(working copy)
@@ -217,4 +217,11 @@
   bool isString(Constant constant) => constant.isString();
   bool isBool(Constant constant) => constant.isBool();
   bool isNull(Constant constant) => constant.isNull();
-}
+
+  bool isSubtype(Compiler compiler, DartType s, DartType t) {
+    if (s.element == compiler.intClass && t.element == compiler.doubleClass) {

[kasperl, 2012/09/14 05:53:34]

Add a comment that explains why this is -- and why we do not need it in the
other direction.

[ngeoffray, 2012/09/14 09:44:56]

Done.

+      return true;
+    }
+    return compiler.types.isSubtype(s, t);
+  }
+}