Revert "Allow LetCont to bind multiple continuations."

pkg/compiler/lib/src/cps_ir/type_propagation.dart

Index: pkg/compiler/lib/src/cps_ir/type_propagation.dart
diff --git a/pkg/compiler/lib/src/cps_ir/type_propagation.dart b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
index 4a3baf74b4b5ad02c25f7d911d2252e8c54f9c4a..c68a91780509aa965fc2eae633b1f084c940ae77 100644
--- a/pkg/compiler/lib/src/cps_ir/type_propagation.dart
+++ b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
@@ -874,4 +874,4 @@ class _AbstractValue<T> {
       return new _AbstractValue.nonConst(typeSystem.join(this.type, that.type));
     }
   }
-}
+}