Support checked mode

pkg/compiler/lib/src/resolution/signatures.dart

Index: pkg/compiler/lib/src/resolution/signatures.dart
diff --git a/pkg/compiler/lib/src/resolution/signatures.dart b/pkg/compiler/lib/src/resolution/signatures.dart
index d3f3264e06a649e3dd7d7797e83ffc431c9934a0..d642a96a309d531e6d2d647063e19efe7c1b7e8a 100644
--- a/pkg/compiler/lib/src/resolution/signatures.dart
+++ b/pkg/compiler/lib/src/resolution/signatures.dart
@@ -366,9 +366,7 @@ class SignatureResolver extends MappingVisitor<FormalElementX> {
       returnType = element.enclosingClass.thisType;
       // Because there is no type annotation for the return type of
       // this element, we explicitly add one.
-      if (compiler.options.enableTypeAssertions) {
-        registry.registerTypeUse(new TypeUse.checkedModeCheck(returnType));
-      }
+      registry.registerTypeUse(new TypeUse.checkedModeCheck(returnType));
     } else {
       AsyncMarker asyncMarker = AsyncMarker.SYNC;
       if (isFunctionExpression) {