dart2js cps: Clean up and avoid processing unreachable code.

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

Index: pkg/compiler/lib/src/cps_ir/type_mask_system.dart
diff --git a/pkg/compiler/lib/src/cps_ir/type_mask_system.dart b/pkg/compiler/lib/src/cps_ir/type_mask_system.dart
index 58d7330cc754175d428d52499d86a7e1c60d0ad0..ed2157241846a385c363cb7c161c306732ced791 100644
--- a/pkg/compiler/lib/src/cps_ir/type_mask_system.dart
+++ b/pkg/compiler/lib/src/cps_ir/type_mask_system.dart
@@ -16,6 +16,7 @@ import '../types/constants.dart' show computeTypeMask;
 import '../universe/selector.dart' show Selector;
 import '../universe/call_structure.dart' show CallStructure;
 import '../world.dart' show World;
+import '../closure.dart' show ClosureFieldElement, BoxLocal, TypeVariableLocal;
 
 enum AbstractBool {
   True, False, Maybe, Nothing
@@ -181,6 +182,14 @@ class TypeMaskSystem {
   }
 
   TypeMask getFieldType(FieldElement field) {
+    if (field is ClosureFieldElement) {
+      // The type inference does not report types for all closure fields.
+      // Box fields are never null.
+      if (field.local is BoxLocal) return nonNullType;
+      // Closure fields for type variables contain the internal representation
+      // of the type (which can be null), not the Type object.
+      if (field.local is TypeVariableLocal) return dynamicType;
+    }
     return inferrer.getGuaranteedTypeOfElement(field);
   }